wealth from oceans

Wealth from Oceans

Wealth from Oceans CSIRO Petroleum Open Report

June 2006

Falling Water Levels in the Latrobe Aquifer, Gippsland Basin

Jim Underschultz, Wayne Bailey,

Reem Freij-Ayoub and Claus Otto

Offshore Aquifer Update, Onshore Fault Seal Analysis, and Preliminary Numerical Simulation

of Coastal Subsidence Risk

For further information contact: Jim Underscuhltz

P.O. Box 1130, Bentley Western Australia, 6102

Australia Tel: +61 8 6436 8747 Fax: +61 8 6436 8555

CONTENTS EXECUTIVE SUMMARY i INTRODUCTION AND BACKGROUNG 1 HYDRODYNAMICS UPDATE 1 Data 2 Methodology 2 Hydraulic Head Distributions 4 Pre Production hydrodynamic System 4 Mid 1980's Hydrodynamic System 6 Mid 1990's Hydrodynamic System 6 Present Day hydrodynamic System 9 Hydrodynamics Update Summary 10 ONSHORE FAULT SEAL ANALYSIS 13 Faults as Barriers to Fluid Flow 13 Seismic Data 15 Onshore Fault Seal Summary 19 COASTAL SUBSIDENCE RISK 21 Objectives 21 Consolidation Theory 22 Methodology 26 Model Assumptions 27 The Analytical Solution Results 30 The Numerical Model 34 Modelling Stages and Input Parameters 34 Numerical Simulation Results for Ultimate Subsidence using SKM

Parameters 35 Numerical Simulation Results for Ultimate subsidence: Sensitivity to

Preconsolidation Stress 39 Sensitivity to Sediment Compressibility 40 Discussion 44 Subsidence Modelling Conclusion and Recommendations 45 GENERAL CONCLUSIONS 48 NEXT PHASE OF WORK PROPOSAL 49 References 50

i

EXECUTIVE SUMMARY The work conducted and reported here, represents the first in a series of research activities aimed at addressing recommendations made in the Hatton report (Hatton et al. 2004) on falling water levels in the Gippsland Basin. This stage of work focused on 3 issues: (1) Uncertainty in certain aspects of the hydrodynamic analysis in the Hatton report due to lack of formation pressure data; 2) Uncertainty in the location and vertical extent of basin forming faults, and 3) Uncertainty in previous subsidence models. Results are summarized as follows.

• Additional pressure data collected for the offshore has both confirmed and sharpened the definition of the region and extent of aquifer pressure depletion. This now includes the area of Kingfish and West Kingfish. While no additional data for the time since 1990 was available, an extrapolation of historic data in two key areas (Fortescue and Kingfish) was conducted to estimate the severity of present day depletion.

• Examination of onshore seismic data has confirmed the location and extent of basin forming fault systems on the boundaries of the Seaspray Depression. Previously, these were suspected to be responsible for compartmentalizing parts of the Latrobe Aquifer system. This confirms that the geographic variation in the degree of pressure depletion observed in the Hatton report is likely related to faulted domains of the Latrobe aquifer. Within each domain, the relative cause of depletion is different. Seismic data frequency and quality is insufficient to perform a complete shale gouge ratio determination on the fault zones for independent quantification of each faults seal capacity.

• First stage simulations of subsidence, with parameters identical to the previous deterministic model (Sinclair Knight Merz, 2001), produced similar but not identical results (the CSIRO model predicted only half the subsidence reported in SKM report). The most likely reason for the discrepancy is ambiguity on the preconsolidation stress. The sensitivity analysis has shown that uncertainty in the preconsolidation stress creates the most variation in predicted total subsidence. This uncertainty may be reduced if burial history curves for the basin can be determined.

In the next phase of work, it is proposed that the numerical subsidence model be expanded to incorporate the entire coastal region described in the Hatton report. Rather than using parameters to match those used by the SKM report, the actual three-D pressure depletion profile characterized in this report will be input to the numerical simulation. Similarly, the actual stratigraphic and structural geology along the coastal zone will be built into the model. Realistic rock properties will be used, and the range of possible values (literature values) will be tested in order to allow for an optimistic and pessimistic prediction of subsidence. The model results will be linked in a Geographic Information System and overlain with a digital elevation model of the land surface, and the predicted future seal level changes and storm wave height data. The result will be a coastal subsidence risk map. In future, if funds are available to collect rock property data, the coastal subsidence risk map can guide the data collection

ii

to areas of highest risk. The numerical simulation shall include the possible mitigating impact of offshore CO2 sequestration that might allow a faster recovery of aquifer pressure. With the three-D numerical model complete, it can be calibrated against observations from the Victorian DPI high resolution GPS monitoring program.

1

INTRODUCTION AND BACKGROUND An investigation and overview of previous work on the falling water levels in the Latrobe Aquifer system of the Gippsland Basin was described by the “Hatton Report” in 2004 (Hatton et al, 2004). It concluded that significant withdrawal of fluid resulted in a complex pattern of pressure decline. This pattern reflects both the structural complexity of the basin as well as the impacts of withdrawals by the mining, irrigation and offshore oil and gas industry. A review of subsidence modelling and monitoring indicated “no unambiguous history of subsidence” except the well-documented subsidence around the coal mines of the Latrobe Valley. To further understand the controls on falling water levels and risks of subsidence and to move towards a mitigation strategy, the Hatton report made the following recommendations.

1. Expand the preliminary analysis and conduct a detailed pressure decline study of the Gippsland Basin using all relevant onshore and offshore wells and historical production and pressure data. A better appreciation of the basin’s structural and lithological complexity is required, followed by a regional modelling study that incorporates this added detail.

2. Review the current groundwater level and formation pressure monitoring network, including an assessment of required refurbishment of wells.

3. Identify critical monitoring wells and install pressure transducers and loggers to measure water levels and pressures at more frequent and regular intervals.

4. Conduct a more comprehensive risk assessment study of potential subsidence in the coastal region, including petrophysical measurements on core linked with further subsidence modelling.

5. Review the option to use satellite-based subsidence monitoring technology.

6. Development of an integrated decision support and groundwater management system for the Gippsland Basin.

In this phase of work, point 1 and parts of point 4, in the above list, have been addressed. The Victorian Department of Primary Industries (Victorian DPI) has reviewed point 5. DPI has determined that satellite-based technology would not work satisfactorily in this situation since insufficient large fixed objects (such as buildings) occur across the region of interest to achieve the desired precision of measurement. Alternatively DPI has embarked on a 5 year monitoring program using high resolution GPS technology. Hydrodynamics Update The hydrodynamic evaluation by Hatton et al. (2004) was based on a limited formation pressure data set due to the high cost of data gathering and entry in the database. This resulted in data gaps in certain onshore and offshore regions. For this report, additional data from 26 wells in key offshore locations were collected and added to the database in order to address some of the data gaps. These data were then used to update and better constrain the

2

hydraulic head maps in the offshore areas. Maps were constructed for the same time slices as reported previously by Hatton et al. (2004). Data In addition to the data available from Hatton et al. (2004) and Underschultz et al. (2003), pressure data from 26 wells were entered into the Petroleum Hydrogeology Group quality controlled pressure database (PHG) described by Otto et al. (2001). These were selected from regions of interest (Veilfin, Fortescue, Kingfish, and Bream oil and gas fields) where data control was previously lacking. A total of 2374 quality-controlled data points were used for this evaluation. Test types include Drillstem Tests (DST), Formation Interval Tests (FITP), Wireline Formation Tests (WFT) and drilling Kicks (KICK). The number of each type of data is shown in Table 1.

Table 1. Statistics on pressure test data.

Test Type Number of data points % of total

DST 55 2.3 FITP 183 7.7 WFT 2135 89.9 KICK 1 0.04

Methodology Standard hydrodynamic approaches to characterizing flow systems in aquifers include the analysis of pressure data, both in vertical profile (e.g. pressure-elevation plot), and within the plane of the aquifer after conversion to hydraulic head. Pressure data are supplemented with formation water analysis and formation temperature data to aid in the evaluation of the flow system. Bachu and Michael (2002), Otto et al. (2001), Bachu (1995), and Dahlburg (1995) provide an overview of hydrodynamic analysis techniques. Evaluation techniques for the culling and analysis of formation water samples are described by Underschultz et al. (2002) and Hitchon and Brulotte (1994). Techniques for the evaluation of formation temperature are described by Bachu et al. (1995) and Bachu and Burwash (1991). The hydrostratigraphy that was adopted by Hatton et al. (2004) and Underschultz et al. (2003) is used in this evaluation (Figure 1) and in this case, only the Upper Latrobe Aquifer is evaluated. Following Hatton et al. (2004), hydraulic head maps are constructed for 4 different times: as close as possible to virgin conditions, the mid 1980’s, the mid 1990’s, and the present day. Depending on the time period, the distribution of data control varies. Pressure data regularly recorded over time from producing wells, and data from wells less than two years old, are confidential. Therefore, the least constrained hydraulic head distribution is the one for the present day.

3

AGES MAJOR UNITS

MiddleP. Tuberculatus

SPORE-POLLEN ZONES

LowerP. Tuberculatus

MiddleN. Asperus

UpperN. Asperus

LowerN. Asperus

P. Asperopolus

Upper L. Balmei

LowerL. Balmei

Upper F. Longus

Lower F. Longus

T. Lilliei

N. Senectus

T. Apoxyexinus

P. Mawsonii

H. Uniforma

P. PannosusAlbian

Ceno-manian

Turonian Coniac.

Santonian

Cam

pani

an M

aast

. P

aleo

cene

Eoc

ene

Olig

ocen

e

Ear

ly L

ate

Lat

e M

iddl

e E

arly

Ear

ly L

ate

Lat

e E

arly

Lat

e E

arly

SEASPRAY GROUP

COBIASUBGROUP

HALIBUTSUBGROUP

GOLDEN BEACH SUBGROUP

EMPERORSUBGROUP

STRZELECKI GROUP

LAT

RO

BE G

RO

UP

GIPPSLAND BASIN STRATIGRAPHYONSHORE OFFSHORE

Lakes Entrance Formation

Balo

ok F

mM

orwe

ll Fm

La

trobe

Va

lley

Subg

roup

SwordfishFormation

Top Latrobe Surfa ce EarlyOligoc eneWedge

MarshallParaconformity

Tur

rum

Traralgon FmBurong Form

a tion

Marlin Unconformity

Gurnard Fm

Mid M. Diversus Marker

Mid Paleocene Marker

Currajung Volcanics

Barracouta KingfishFormation

Formation

Opah Channe l Opah Fm

Flounder Fm

Latro

be U

ncon

form

ity

MackerelFormation

Kate Shale

VoladorFormation

AnemoneFormation

Seahorse Unconformity

Campanian Volcanics

Longtom Unconformity NORTH

ChimaeraFormation

SOUTH

Kersop Arkose Kipper Shale Curlip Fm

Admiral Fm

Otway Uncon formity

Korumburra Subgroup

GIPPSLAND BASIN HYDROSTRATIGRAPHY

Upper Latrobe Aquifer System

Lower Latrobe Aquifer System

Mid Latrobe Aquitard System

Figure 1. Stratigraphic and hydrostratigraphic nomenclature.

4

Hydraulic Head Distributions The aquifer system was characterized for each time period by comparing the date at which a formation pressure was measured with the date at which various fields came online (production began). Data which pre-date production and certain data that post-date production but are located geographically far away from the site of production, were selected and used to characterize the virgin state of the flow system. Data used for characterizing the other time periods are simply the values that were measured within the desired time period. Data are flagged on the maps either by a bright or dim yellow background; the former are used directly as control points for the contour map, the latter as indirect constraints to the contour distribution. For example, while a 1989 measurement cannot be used as a direct value for the mid 1980’s contour map, since the pressure could have dropped between the mid 1980’s and 1989 when the pressure was measured, it can be used as a minimum value for constraining the contours i.e. the pressure must have been higher in the mid 1980’s than what was measured in 1989 since no enhanced recovery methods are used. Since new pressure data added for this update are from the offshore regions, there are no changes to the onshore part of the characterisation of Hatton et al. (2004). As such, only the offshore regions are described in this report. For a description of the onshore regions, refer to Hatton et al. (2004). For each map offshore gas fields are shown in pink and oil fields in green. Onshore coal mines are shown in green. Pre Production Hydrodynamic System The distribution of virgin hydraulic head for the Upper Latrobe Aquifer System is shown in Figure 2. At each well the best available data are posted (relative to sea level). To assist in interpreting the hydraulic head distribution, the spud date (year) of each well is indicated and the year each field began producing is indicated (e.g. Barracouta 69). Occasionally, in the absence of well based pressure data, the pre-production pool pressure is converted to hydraulic head and used to constrain the hydraulic head distribution. It was assumed that the pre-production pool pressure was valid at the time of the earliest well drilled for the field. In some cases there is a large time gap between field discovery and initiation of production. In these situations the quoted initial field pressure was

5

Latrobe subcrop

Latrobe subcrop

Kingfish 71West Kingfish 82

Bream 88

Bream B 96

Dolphin 90

Tarwhine 90

Seahorse 90

Mackerel 77

Corbia 79

Fortescue 83

Marlin 69

Snapper 81

Moonfish 97

Barracouta 69

Whiting 89

Angelfish

Tuna 79

Halibut 70

Sole Field50km

RosedaleTraralgon

Morwell

Churchill

Sale

Maffra

Approx. Edge of Gippsland Basin

Approx. Edge of G

ippsland Ba

sin

0 25km

Scale

N

Rosedale Fault System

Foster Fault System

Darriman Fault System

Lake Wellington Fault System

CentralDeep

SeasprayDepression

LatrobeValley

North Strzelecki Terrace

South Strzelecki Terrace

Northern Platform

? ?

-46m head

43m head

22m head

47m head45m head

52m head

34m head

59m head

59m head

64m headFlounder

58m head

-2m head

40

45

14 17

12

37

18

10 10

38

77

59

8

3

27

86

36

86

124

53

53

-74

-45

21

22

63

48

15

15

-3

-6

-5-9

-11

-49

-17

-24

-23

-23

-44

-40

34-12

-23

-1

25

34

33

153

2

1

4734

13412

34

60

2630

15

21

42

4855

4682

23

24

25

70

11

97

5

(84)

(83)

(85)

(75)

(80)(79)

(79)(83)

(82)

(93)

(85)

(89)

(81)

Head (used directly)Head (used indirectly)

4811

2550

(86)(86)

5040

40

40

30

50

80

40

30

20

70

60 50

60

40

60

70

50

60

70

80

60

80

Upper Latrobe Aquifer System - Virgin Hydraulic Head (m) Distribution

(’85)(’89)

(’67)

(’77)

(’85)

(’69)

(’65)

(’64)

(’83)

(’83)(’82)

(’89)(’78)

(’82)(’69)

(’83)

(’85)

(’81)

(’85)(’85)

(’83)

(’68)

(’89)(’70)

(’92)(’94)

(’89)(’74) (’83)

(’69)

(’74)

(’85)

(’92)

(’95)

(’95)

(’66)

(’66)

(’65) (’73)

(’84)

(’78)(’78)

(’79)

(’78)

(’83)(’94)

(’85)

(’89)

(’67)

(’67)

(’89)

(’82)

(’82)(’81)

(’82)

(’82)

(’83)

(’84)

(’89)

(’68)

(’92)

(’92)

(’67)

(’68)

(’77)

(’77)

(’72)

(’74)

(’73)

(’69)(’85)

(’78)

(’76)

(’84)

(’65)

(’83)

(’82)

(’82)

(’82)

(’69)

(’81)

(’70)

(’65)

(’66)

(’61)

(’81)

(’69)

Fortescue 3

Turrum 2

Seahorse 1Seahorse 2

West Seahorse 1Harlequin 1

Barracouta 1

Barracouta 4Barracouta 2

Barracouta 5

Barracouta 3Whiptail 1AMulloway 1

Tarwhine 1

Luderick 1

Bream 5Bream 4

Bream 2Bream 1

Bream 3Bream A6

Bream B9A

Edina 1

Omeo 1

Omeo 2A

Tarra 1

Bullseye 1

Perch 1

Kyarra 1AWyrallah 1

Tommyruff 1

Broadbill 1Woodside 1

Woodside 12

Woodside 2

Woodside 4

Sunday Island 1

Yarram 1

Westrailian 1

Gippsland 3

Woranga 12

Woranga 16

147174

Welshpool 31

Woranga 15

Stringy Bark 1

Darriman 1

Salt lake 1

Lake Reeve 1

Cars Creek 1

Signal Hill 1

Bellbird 1

Coolungoolun 93

Denison 341

Stradbroke 51

Holey Plains 177

Tong Bong 167

Rosedale 1

Lake Kakydra 1

Nuntin 1

Avon 1

Gippsland 4

Nuntin 2

Sale 13Sale 15001

Yeerung 1 Meerlieu 15001

Stradbroke 4

Tom’s Creek 3

Stradbroke 1

Nindoo 1

SW Bairnsdale 1

Bundalaguah 4

Bundalaguah 10

Narracan 3607

Meerlieu Bore 1

Glenmaggie 1

Midfield 1

Wulla 2

Glencoe South 7

Wulla Wullock 5

Wulla Wullock 7

Glencoe 7

McGreesh 1

Stradbroke 9

Stradbroke 15

Gilford 14

Gilford 6

Merridian 1

Crossroads 1

Darriman 3

Boodyarn 4/6

Woodside South 1

Palmer 1

Dolphin 1

Blenny 1

Torsk 1

Speke 1

Whiting 2

Whiting 1

Cod 1

Swordfish 1

Veilfin 1

Snapper 4Snapper 5

Snapper 6

Snapper 1

Snapper 2

Snapper 3

Moonfish 1/ST1Moonfish 2

Emperor 1

Cuttlefish 1

Albatross 1

Colquhoun 2

Colquhoun North 1

Gannet 1

East Nowa 1

Dome 4

Sweetlips 1/ST1

Remora 1

Sunfish 1

Turrum 1

Marlin 1

Marlin 3

Marlin 2Turrum 3

Marlin 4

Turrum 4

West Fortescue 1

Fortescue 1

Fortescue 4

Fortescue 2

Halibut 2Halibut 1

Cobia 2Cobia 1Rockling 1

Wirrah 3Wirrah 1

Wirrah 2

Golden Beach 1A

Drummer 1

Teraglin 1

Pilotfish 1

Mackerel 1

Yellowtail 1

Selene 1

Hermes 1

Angler 1

Anemone 1A

Helios 1

Kingfish 5

Kingfish 1Kingfish 2

Kingfish 3Kingfish 8ST1

Kingfish 9

East Kingfish 1

Yellowtail 2

Mackerel 4

Flounder 1

Flounder 6ST1

Trevally 1

Angelfish 1

Batfish 1

Tuna 4

Longtom 1ST1

Sperm Whale 1

Patricia 1

Baleen 1

East End Bore 1

Morwong1

Wrasse 1

Flounder 4

East Halibut 1Trumpeter 1

Dutson Downs 1

Dulung Along 1

Golden Beach West 1

Seacombe South 1

Seacombe 7East Reeve 1

East Seacombe 1

Pelican Point 1

Point Addis 2

Duck bay 1

Bairnsdale 5Bairnsdale 15005

Bengworden South 1

Bengworden 2

Moormurrng 5

Moormurrng Bore 1

Forge Creek Bore 4

Coolgulmerang 1

Coolgulmerang 2

Wrixondale 1

Wellington Park 1

Turrum 6 St1

Barracouta 5

Snapper 2

Turrum 5

Snapper A21

Figure 2. Pre-Production hydraulic head distribution for the Upper Latrobe Aquifer System.

6

only at the discovery date and not used as a control point at the later date (start of production). Field pressure history data, if available, could help define how long the initial pool pressure could be used post discovery as a valid pressure constraint of the aquifer at each pool. As described in Hatton et al. (2004) and Underschultz et al. (2003), there are competing basinal driving forces in the Gippsland Basin. High hydraulic head extending eastwards from onshore subcrop reflects gravity driven freshwater recharge from the west (Figure 2). This is particularly prominent within the boundaries of the Seaspray Depression and the western part of the offshore Central Deep. Compaction driven dewatering of the offshore sedimentary pile is expressed as regions of high hydraulic head (roughly those areas greater than 50m head). In the central part of the Central Deep, there is a region of less than 30m of hydraulic head that forms a sink into which the formation water flows. It appears that this sink is connected to the Darriman Fault system on the southern edge of the Central Deep. It is postulated that formation water may discharge from the Latrobe aquifer up the Darriman Fault System either to an upper aquifer or even to the seabed. The new data added as part of this study have revealed more detail to the geometry of the low hydraulic head sink in the Central Deep. It has several arms or troughs of low hydraulic head which extend to the north and east. These tend to pass between the main hydrocarbon pools. A major trough extends northwards between the Snapper and Marlin fields and a second one extends eastwards between the Fortescue and Kingfish fields.

Mid 1980’s Hydrodynamic System

The distribution of hydraulic head for the Upper Latrobe Aquifer System in the mid 1980’s is shown in Figure 3. It is an update of the map described by Hatton et al. (2004) with newly added data to constrain contours in the vicinity of the Kingfish and Yellowtail field areas. These new data show that by the mid 1980’s, production from the Halibut, Fortescue, Corbia, Mackerel and Kingfish fields has resulted in a localized decrease in the hydraulic head surface for the Upper Latrobe Aquifer System. In the area of Fortescue, this hydraulic head reaches about -50m and about -20m at Kingfish. At this time, the most intense part of the depression is localized, with the Selene-1, Angler-1 and Anemone-1A wells to the southeast relatively unaffected, having hydraulic head still at about 50m. The area on the southern edge of the Central Deep previously described as a possible discharge region (Figure 2) is of particular interest. By the mid 1980’s the flow direction in this region is reversed and the previously discharge related zone along the Darriman fault bend may now be contributing as a recharge to the Latrobe Aquifer system.

Mid 1990’s Hydrodynamic System

The distribution of hydraulic head for the Upper Latrobe Aquifer System in the mid 1990’s is shown in Figure 4. The dashed hydraulic head contours in Figure 4 indicate that data control is limited and consequently uncertainty in the hydraulic head distribution is high. It is an update of the map described by Hatton et al. (2004) with newly added data to constrain contours in the vicinity

7

Latrobe subcrop

Latrobe subcrop


Bream 88

Bream B 96

Dolphin 90

Tarwhine 90

Seahorse 90

Mackerel 77

Corbia 79

Fortescue 83

Marlin 69

Snapper 81

Moonfish 97

Barracouta 69

Whiting 89

Angelfish

Tuna 79

Halibut 70

Sole Field50km

RosedaleTraralgon

Morwell

Churchill

Sale

Maffra


Approx. Edge of G

ippsland Ba

sin

0 25km

Scale

N


Foster Fault System



CentralDeep

SeasprayDepression

LatrobeValley



Northern Platform

? ?

50

40

30

30

20

20

20

100

3040

10

10 -10 -50

-20

-20

030

2010

-10

-20-30

-50

0

6070

60

Upper Latrobe Aquifer System - Inferred Hydraulic Head (m) Distribution for 1986


4811

17

12

20

23 11

18

-45

21

-3

-49

57

73

-21-20

51

-5

-22

38

-23

34

32

34

26

11

542

41

49

82

24

14

20

25

24

50

26

66

25

21

25

(86)

(86)

(86)

(86)

(86)(86)

(86)

(86)

(86)

(86)

(86)

(89)

(86)

(86)

(86)

(89)

Contours from the mine site area are taken from“regional groundwater investigation of theLatrobe Valley. Stage 2 mathematical modellingstudies V 2. Rept no Dd187. Oct 1983. R. Evans

(’85)(’89)

(’67)

(’77)

(’85)

(’69)

(’65)

(’64)

(’83)

(’83)(’82)

(’89)(’78)

(’82)(’69)

(’83)

(’85)

(’81)

(’85)(’85)

(’83)

(’68)

(’89)(’70)

(’92)(’94)

(’89)(’74) (’83)

(’69)

(’74)

(’85)

(’92)

(’95)

(’95)

(’66)

(’66)

(’65) (’73)

(’84)

(’78)(’78)

(’79)

(’78)

(’83)(’94)

(’85)

(’89)

(’67)

(’67)

(’89)

(’82)

(’82)(’81)

(’82)

(’82)

(’83)

(’84)

(’89)

(’68)

(’92)

(’92)

(’67)

(’68)

(’77)

(’77)

(’72)

(’74)

(’73)

(’69)(’85)

(’78)

(’76)

(’84)

(’65)

(’83)

(’82)

(’82)

(’82)

(’69)

(’81)

(’70)

(’65)

(’66)

(’61)

(’81)

(’69)

Fortescue 3

Turrum 2



Barracouta 1


Barracouta 5


Tarwhine 1

Luderick 1

Bream 5Bream 4

Bream 2Bream 1

Bream 3Bream A6

Bream B9A

Edina 1

Omeo 1

Omeo 2A

Tarra 1

Bullseye 1

Perch 1

Kyarra 1AWyrallah 1

Tommyruff 1


Woodside 12

Woodside 2

Woodside 4

Sunday Island 1

Yarram 1

Westrailian 1

Gippsland 3

Woranga 12

Woranga 16

147174

Welshpool 31

Woranga 15

Stringy Bark 1

Darriman 1

Salt lake 1

Lake Reeve 1

Cars Creek 1

Signal Hill 1

Bellbird 1

Coolungoolun 93

Denison 341

Stradbroke 51

Holey Plains 177

Tong Bong 167

Rosedale 1

Lake Kakydra 1

Nuntin 1

Avon 1

Gippsland 4

Nuntin 2

Sale 13Sale 15001


Stradbroke 4

Tom’s Creek 3

Stradbroke 1

Nindoo 1

SW Bairnsdale 1

Bundalaguah 4

Bundalaguah 10

Narracan 3607

Meerlieu Bore 1

Glenmaggie 1

Midfield 1

Wulla 2

Glencoe South 7

Wulla Wullock 5

Wulla Wullock 7

Glencoe 7

McGreesh 1

Stradbroke 9

Stradbroke 15

Gilford 14

Gilford 6

Merridian 1

Crossroads 1

Darriman 3

Boodyarn 4/6

Woodside South 1

Palmer 1

Dolphin 1

Blenny 1

Torsk 1

Speke 1

Whiting 2

Whiting 1

Cod 1

Swordfish 1

Veilfin 1

Snapper 4Snapper 5

Snapper 6

Snapper 1

Snapper 2

Snapper 3


Emperor 1

Cuttlefish 1

Albatross 1

Colquhoun 2

Colquhoun North 1

Gannet 1

East Nowa 1

Dome 4

Sweetlips 1/ST1

Remora 1

Sunfish 1

Turrum 1

Marlin 1

Marlin 3

Marlin 2Turrum 3

Marlin 4

Turrum 4

West Fortescue 1

Fortescue 1

Fortescue 4

Fortescue 2

Halibut 2Halibut 1


Wirrah 3Wirrah 1

Wirrah 2

Golden Beach 1A

Drummer 1

Teraglin 1

Pilotfish 1

Mackerel 1

Yellowtail 1

Selene 1

Hermes 1

Angler 1

Anemone 1A

Helios 1

Kingfish 5



Kingfish 9

East Kingfish 1

Yellowtail 2

Mackerel 4

Flounder 1

Flounder 6ST1

Trevally 1

Angelfish 1

Batfish 1

Tuna 4

Longtom 1ST1

Sperm Whale 1

Patricia 1

Baleen 1

East End Bore 1

Morwong1

Wrasse 1

Flounder 4


Dutson Downs 1

Dulung Along 1

Golden Beach West 1

Seacombe South 1


East Seacombe 1

Pelican Point 1

Point Addis 2

Duck bay 1


Bengworden South 1

Bengworden 2

Moormurrng 5

Moormurrng Bore 1

Forge Creek Bore 4

Coolgulmerang 1

Coolgulmerang 2

Wrixondale 1

Wellington Park 1

Turrum 6 St1

Barracouta 5

Snapper 2

Turrum 5

Snapper A21

Figure 3. Mid 1980’s hydraulic head distribution for the Upper Latrobe Aquifer System.

8

Latrobe subcrop

Latrobe subcrop


Bream 88

Bream B 96

Dolphin 90

Tarwhine 90

Seahorse 90

Mackerel 77

Corbia 79

Fortescue 83

Marlin 69

Snapper 81

Moonfish 97

Barracouta 69

Whiting 89

Angelfish

Tuna 79

Halibut 70

Sole Field50km

RosedaleTraralgon

Morwell

Churchill

Sale

Maffra


Approx. Edge of G

ippsland Ba

sin

0 25km

Scale

N


Foster Fault System



CentralDeep

SeasprayDepression

LatrobeValley



Northern Platform

? ?

-46m head

43m head

-16m head

34m head

59m head

-2m head

14

7

38

-74

-38

-38

22

-44

-40

-12-23

-1

20

21

16

11

11

14

60

32

20

23

45

2120

14

19

7

29

40

80

15

-100

-50

-40

-20

-10

-10

0

-30

-60-70

-30

-10

0

-10

20

Upper Latrobe Aquifer System - Inferred Hydraulic Head (m) Distribution for mid 1990’s


4811

(95)

(95)

(95)

(95)

(95)

(95)

(95)

(95)

(95)

(95)

(95)

(95)

(95)

(95)

(95)

(95)

(95)

(95)

(95)

(95)

(97)

20 2010

10 0

6070

40

30

20

20

10

10

20

50

60

Locally depressedhydraulic head surfacealso noted by SKM report(January 2004)

-10

-20

0

-30

-50-50

Data from the mine site area is taken from“Latrobe Valley regional groundwater and landsurface monitoring report five year review as atJune 2000. # 1000/8505/99 Geo-eng pty ltd.

(’85)(’89)

(’67)

(’77)

(’85)

(’69)

(’65)

(’64)

(’83)

(’83)(’82)

(’89)(’78)

(’82)(’69)

(’83)

(’85)

(’81)

(’85)(’85)

(’83)

(’68)

(’89)(’70)

(’92)(’94)

(’89)(’74) (’83)

(’69)

(’74)

(’85)

(’92)

(’95)

(’95)

(’66)

(’66)

(’65) (’73)

(’84)

(’78)(’78)

(’79)

(’78)

(’83)(’94)

(’85)

(’89)

(’67)

(’67)

(’89)

(’82)

(’82)(’81)

(’82)

(’82)

(’83)

(’84)

(’89)

(’68)

(’92)

(’92)

(’67)

(’68)

(’77)

(’77)

(’72)

(’74)

(’73)

(’69)(’85)

(’78)

(’76)

(’84)

(’65)

(’83)

(’82)

(’82)

(’82)

(’69)

(’81)

(’70)

(’65)

(’66)

(’61)

(’81)

(’69)

Fortescue 3

Turrum 2



Barracouta 1


Barracouta 5


Tarwhine 1

Luderick 1

Bream 5Bream 4

Bream 2Bream 1

Bream 3Bream A6

Bream B9A

Edina 1

Omeo 1

Omeo 2A

Tarra 1

Bullseye 1

Perch 1

Kyarra 1AWyrallah 1

Tommyruff 1


Woodside 12

Woodside 2

Woodside 4

Sunday Island 1

Yarram 1

Westrailian 1

Gippsland 3

Woranga 12

Woranga 16

147174

Welshpool 31

Woranga 15

Stringy Bark 1

Darriman 1

Salt lake 1

Lake Reeve 1

Cars Creek 1

Signal Hill 1

Bellbird 1

Coolungoolun 93

Denison 341

Stradbroke 51

Holey Plains 177

Tong Bong 167

Rosedale 1

Lake Kakydra 1

Nuntin 1

Avon 1

Gippsland 4

Nuntin 2

Sale 13Sale 15001


Stradbroke 4

Tom’s Creek 3

Stradbroke 1

Nindoo 1

SW Bairnsdale 1

Bundalaguah 4

Bundalaguah 10

Narracan 3607

Meerlieu Bore 1

Glenmaggie 1

Midfield 1

Wulla 2

Glencoe South 7

Wulla Wullock 5

Wulla Wullock 7

Glencoe 7

McGreesh 1

Stradbroke 9

Stradbroke 15

Gilford 14

Gilford 6

Merridian 1

Crossroads 1

Darriman 3

Boodyarn 4/6

Woodside South 1

Palmer 1

Dolphin 1

Blenny 1

Torsk 1

Speke 1

Whiting 2

Whiting 1

Cod 1

Swordfish 1

Veilfin 1

Snapper 4Snapper 5

Snapper 6

Snapper 1

Snapper 2

Snapper 3


Emperor 1

Cuttlefish 1

Albatross 1

Colquhoun 2

Colquhoun North 1

Gannet 1

East Nowa 1

Dome 4

Sweetlips 1/ST1

Remora 1

Sunfish 1

Turrum 1

Marlin 1

Marlin 3

Marlin 2Turrum 3

Marlin 4

Turrum 4

West Fortescue 1

Fortescue 1

Fortescue 4

Fortescue 2

Halibut 2Halibut 1


Wirrah 3Wirrah 1

Wirrah 2

Golden Beach 1A

Drummer 1

Teraglin 1

Pilotfish 1

Mackerel 1

Yellowtail 1

Selene 1

Hermes 1

Angler 1

Anemone 1A

Helios 1

Kingfish 5



Kingfish 9

East Kingfish 1

Yellowtail 2

Mackerel 4

Flounder 1

Flounder 6ST1

Trevally 1

Angelfish 1

Batfish 1

Tuna 4

Longtom 1ST1

Sperm Whale 1

Patricia 1

Baleen 1

East End Bore 1

Morwong1

Wrasse 1

Flounder 4


Dutson Downs 1

Dulung Along 1

Golden Beach West 1

Seacombe South 1


East Seacombe 1

Pelican Point 1

Point Addis 2

Duck bay 1


Bengworden South 1

Bengworden 2

Moormurrng 5

Moormurrng Bore 1

Forge Creek Bore 4

Coolgulmerang 1

Coolgulmerang 2

Wrixondale 1

Wellington Park 1

Turrum 6 St1

Barracouta 5

Snapper 2

Turrum 5

Snapper A21

Figure 4. Mid 1990’s hydraulic head distribution for the Upper Latrobe Aquifer System.

9

of the Kingfish and Yellowtail field areas. Whilst there is less data control in the mid 1990’s than the mid 1980’s, the mid 1980’s map can be used as a guide for the general shape of the hydraulic head distribution in the mid 1990’s. This is used in combination with a few control points from Halibut and Kingfish development wells, to estimate the mid 1990’s hydraulic head distribution. With continued oil production, the hydraulic head decrease centred on the Fortescue to Kingfish fields becomes greater. In the Fortescue area, the hydraulic head is below -70m and less than -40m in the Kingfish area. The area of low hydraulic head also now extends north and west to include the Marlin, Snapper and Barracouta fields. Regionally, there is a continued slow decrease in the hydraulic head of the Upper Latrobe Aquifer System with data at locations such as Dolphin dropping from 26m in 1989 to 7m of hydraulic head in 1997.

Present Day Hydrodynamic System

There are almost no present-day offshore pressure data available in the public domain. In the absence of evidence to the contrary, it is reasonable to assume that the trend of decreasing hydraulic head with time observed during the first 20 years of hydrocarbon production continues to the current day. It is then possible to estimate the present day hydraulic head distribution using the mapped distribution for the mid 1990’s and the observed trend in decreasing hydraulic head with time during the 1990’s. Ideally, pressure depletion with time should be plotted for a single well location that has had multiple formation pressure measurements over time from the same reservoir interval. The publicly available data used in this report do not include multiple formation pressure measurements made over the production life of the wells. However, a pressure-time plot can be constructed for an entire field, if there have been several development wells drilled and pressure tested during the course of production. It is important that only data from within the field boundary are used, since the pressure depletion profile could change with geographic position away from the production wells. This procedure assumes that the field being examined is not subject to significant production compartmentalization. In order to reduce uncertainty, all pressure values were first converted to a common datum. Sufficient pressure data are available from well completion reports for the Fortescue area (Fortescue, Halibut, Corbia and Mackerel) and the Kingfish area (Kingfish and West Kingfish) to create pressure versus time profiles for each area. Data were corrected to a datum of 2350mSS True Vertical Depth (TVD), the average depth of the reservoir in the region of interest. Formation pressure values were corrected in a staged procedure. If the pressure value to be extrapolated represents formation water, it is corrected according to the water hydrostatic gradient (1.40 psi/m) to the datum elevation. If the pressure value to be corrected represents an oil column, it is first corrected down an oil pressure gradient (0.902 psi/m) to the estimated free water level, then, it is corrected back to the reference datum according to the water hydrostatic gradient (1.40 psi/m). Only one representative pressure value

10

is taken from each well. For example, in the case of wireline test data where a number of pressure measurements are available, a single representative pressure measurement, preferably from the formation water leg, is used to extrapolate a pressure to the 2350mSS datum. A pressure versus time graph was constructed for the two areas (Figure 5). The pressure-time profile for the Kingfish area is different than that for the Fortescue area (Figure 5), with the Fortescue area showing a more rapid decrease in formation pressure with time. This is reflected in the hydraulic head distribution maps of the mid 1980’s (Figure 3) and mid 1990’s (Figure 4) that show the hydraulic head to be lower at Fortescue than at Kingfish. To estimate the formation pressure expected in the Upper Latrobe Aquifer System at the Kingfish and Fortescue areas today and into the future, the pressure-time profile has been extrapolated to 2020. This assumes that the production rates since the mid 1990’s have not changed substantially and that there has been no significant reservoir stimulation. The estimated hydraulic head distribution for present day (2005) is shown in Figure 6. The present day map shows a continued decrease of hydraulic head particularly around the Kingfish and Fortescue areas. Contours in the offshore are displayed as dashed lines to indicate that the hydraulic head distribution is only an estimate based on extrapolation of pressure trends. Hydrodynamics Update Summary New pressure data added from 26 wells in the offshore part of the study area has been incorporated to the hydrodynamic model. The new data define more detail to the basin centred trough of low hydraulic head and help to constrain the extent of pressure depletion in the vicinity of the Mackerel, Kingfish, West Kingfish and Bream oil and gas field areas. Discharge of the Latrobe Aquifer system is postulated to occur on the southwestern edge of the Central Deep possibly associated with a complicated fault intersection and fault bend of the Darriman Fault System. By the mid 1980’s, the hydrodynamic model suggests that the flow is reversed towards the hydraulic head low (sink) centred at Fortescue and Kingfish, in which case the previous discharge zone may from the mid 1980’s until today contribute as recharge.

11

21.5

22.0

22.5

23.0

23.5

24.0

24.5

Year of Measurement

Pres

sure

cor

rect

ed to

235

0 m

TVDS

S (M

Pa)

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

2020

Kingfish OilfieldFortescue Oilfield

Pressure Drawdown Referenced to 2350 mTVDSS

Estm

ated

ran

ge a

t yea

r 202

0

21.5 MPa

22.0 MPa21.9 MPa

22.4 MPa-71m head

-105m head-122m head

-160m head

Figure 5. Pressure versus time for the Kingfish and Fortescue field area.

12

Latrobe subcrop

Latrobe subcrop


Bream 88

Bream B 96

Dolphin 90

Tarwhine 90

Seahorse 90

Mackerel 77

Corbia 79

Fortescue 83

Marlin 69

Snapper 81

Moonfish 97

Barracouta 69

Whiting 89

Angelfish

Tuna 79

Halibut 70

Sole Field50km

RosedaleTraralgon

Morwell

Churchill

Sale

Maffra


Approx. Edge of G

ippsland Ba

sin

0 25km

Scale

N


Foster Fault System



CentralDeep

SeasprayDepression

LatrobeValley



Northern Platform

? ?

77

63

24

21

33

10

15

15

14

12

13

2

40

5

3

7

9

7

5

4

(03)

(03)

(03)

(03)

(03)

(04)

(04)

(03)

(03)

(04)

(04)

(03)

(04)

(04)

(04)

(04)

(04)

(04)

(04)

(04)

20

10

10

-10

-20

0

6070

40

30

2010

10

5060

Data from the mine site area is taken from“Latrobe Valley regional groundwater and landsurface monitoring report five year review as atJune 2000. # 1000/8505/99 Geo-eng pty ltd.

Upper Latrobe Aquifer System - Inferred Hydraulic Head (m) Distribution for 2004


48

-71

-122

11

-30

-50

-50

-160

-50

-10

0

-10

-20-40

-20

-30

-70

(’85)(’89)

(’67)

(’77)

(’85)

(’69)

(’65)

(’64)

(’83)

(’83)(’82)

(’89)(’78)

(’82)(’69)

(’83)

(’85)

(’81)

(’85)(’85)

(’83)

(’68)

(’89)(’70)

(’92)(’94)

(’89)(’74) (’83)

(’69)

(’74)

(’85)

(’92)

(’95)

(’95)

(’66)

(’66)

(’65) (’73)

(’84)

(’78)(’78)

(’79)

(’78)

(’83)(’94)

(’85)

(’89)

(’67)

(’67)

(’89)

(’82)

(’82)(’81)

(’82)

(’82)

(’83)

(’84)

(’89)

(’68)

(’92)

(’92)

(’67)

(’68)

(’77)

(’77)

(’72)

(’74)

(’73)

(’69)(’85)

(’78)

(’76)

(’84)

(’65)

(’83)

(’82)

(’82)

(’82)

(’69)

(’81)

(’70)

(’65)

(’66)

(’61)

(’81)

(’69)

Fortescue 3

Turrum 2



Barracouta 1


Barracouta 5


Tarwhine 1

Luderick 1

Bream 5Bream 4

Bream 2Bream 1

Bream 3Bream A6

Bream B9A

Edina 1

Omeo 1

Omeo 2A

Tarra 1

Bullseye 1

Perch 1

Kyarra 1AWyrallah 1

Tommyruff 1


Woodside 12

Woodside 2

Woodside 4

Sunday Island 1

Yarram 1

Westrailian 1

Gippsland 3

Woranga 12

Woranga 16

147174

Welshpool 31

Woranga 15

Stringy Bark 1

Darriman 1

Salt lake 1

Lake Reeve 1

Cars Creek 1

Signal Hill 1

Bellbird 1

Coolungoolun 93

Denison 341

Stradbroke 51

Holey Plains 177

Tong Bong 167

Rosedale 1

Lake Kakydra 1

Nuntin 1

Avon 1

Gippsland 4

Nuntin 2

Sale 13Sale 15001


Stradbroke 4

Tom’s Creek 3

Stradbroke 1

Nindoo 1

SW Bairnsdale 1

Bundalaguah 4

Bundalaguah 10

Narracan 3607

Meerlieu Bore 1

Glenmaggie 1

Midfield 1

Wulla 2

Glencoe South 7

Wulla Wullock 5

Wulla Wullock 7

Glencoe 7

McGreesh 1

Stradbroke 9

Stradbroke 15

Gilford 14

Gilford 6

Merridian 1

Crossroads 1

Darriman 3

Boodyarn 4/6

Woodside South 1

Palmer 1

Dolphin 1

Blenny 1

Torsk 1

Speke 1

Whiting 2

Whiting 1

Cod 1

Swordfish 1

Veilfin 1

Snapper 4Snapper 5

Snapper 6

Snapper 1

Snapper 2

Snapper 3


Emperor 1

Cuttlefish 1

Albatross 1

Colquhoun 2

Colquhoun North 1

Gannet 1

East Nowa 1

Dome 4

Sweetlips 1/ST1

Remora 1

Sunfish 1

Turrum 1

Marlin 1

Marlin 3

Marlin 2Turrum 3

Marlin 4

Turrum 4

West Fortescue 1

Fortescue 1

Fortescue 4

Fortescue 2

Halibut 2Halibut 1


Wirrah 3Wirrah 1

Wirrah 2

Golden Beach 1A

Drummer 1

Teraglin 1

Pilotfish 1

Mackerel 1

Yellowtail 1

Selene 1

Hermes 1

Angler 1

Anemone 1A

Helios 1

Kingfish 5



Kingfish 9

East Kingfish 1

Yellowtail 2

Mackerel 4

Flounder 1

Flounder 6ST1

Trevally 1

Angelfish 1

Batfish 1

Tuna 4

Longtom 1ST1

Sperm Whale 1

Patricia 1

Baleen 1

East End Bore 1

Morwong1

Wrasse 1

Flounder 4


Dutson Downs 1

Dulung Along 1

Golden Beach West 1

Seacombe South 1


East Seacombe 1

Pelican Point 1

Point Addis 2

Duck bay 1


Bengworden South 1

Bengworden 2

Moormurrng 5

Moormurrng Bore 1

Forge Creek Bore 4

Coolgulmerang 1

Coolgulmerang 2

Wrixondale 1

Wellington Park 1

Turrum 6 St1

Barracouta 5

Snapper 2

Turrum 5

Snapper A21

Figure 6. Estimated hydraulic head for the Upper Latrobe aquifer in 2004.

13

ONSHORE FAULT SEAL ANALYSIS Amongst the recommendations of Hatton et al. (2004) was a need to better constrain the location and role of faults in controlling the hydrodynamic system and compartmentalisation. This report outlines the appraisal of suitable data for a fault seal evaluation. The principal aims were to review onshore public domain three-D seismic data and well data to determine if they were of sufficient quality to:

Map the positions and displacements along the major Gippsland Basin fault zones (principally the Darriman and Rosedale fault systems);

Attempt a fault seal assessment. Faults as Barriers to Fluid Flow A fault zone can impact groundwater flow by exhibiting either higher or lower permeable porous medium than the aquifer rock matrix. When the fault has lower permeability it can form a barrier to fluid flow. The two main categories of fault seals are:

1. Juxtaposition seals – juxtaposition of impermeable rocks (e.g. shales, mudstones) against the aquifer;

2. Fault rock seals – fault rocks that ‘coat’ the fault surface and possess lower permeability than the aquifer. Fault rock seals comprise one or more of the following types, all of which cause a decrease in permeability through porosity reduction.

Cataclasis – comminution resulting in reduction in grain-size; Deformation induced porosity collapse – disaggregation,

compaction, grain boundary sliding; Diagenesis – including cementation and the generation of

diagenetic reaction products that occlude pore spaces; Shale gouge – (aka shale or clay smears) form where

shale/clay is incorporated into the fault rock by mechanical processes (e.g. abrasion, smearing and injection (Smith, 1980; Lindsay et al. 1993).

To appraise faults as seals, stratigraphic juxtapositions are determined and mapped on to fault surfaces to constrain across-fault stratigraphic juxtapositions (Figure 7). In the subsurface, this commonly requires seismic data to map the fault surface in three-dimensions and well data to define the stratigraphic subdivisions.

14

Figure 7. Method of fault seal assessment. 1. Interpret seismic data; tie stratigraphy from well data to interpreted horizons. 2. Grid the fault in three-D and determine displacements from seismic reflector (horizon) offsets. 3. Map stratigraphic juxtapositions onto fault. 4. Calculate fault seal from throw and sequence composition (see Figure 8).

The hydraulic behaviour of faults, where the aquifer is self-juxtaposed or juxtaposed against another aquifer unit, relies on the fault rock permeability. Outcrop analysis of fault zones demonstrates that the distribution and intensity of fault rock types (e.g. cataclasites, breccias) and diagenetic products (e.g. cements) are complex and inherently unpredictable; the notable exception is fault rock shale content (e.g. Childs et al. 1997; Foxford et al. 1998; Yielding, 2000). Fault rock shale or clay content correlates negatively with pore throat sizes (Gibson 1998) and permeability (Gibson 1998; Sperrevik et al. 2002). Therefore, most techniques employed by the petroleum industry aimed at predicting fault hydraulic properties focus on estimating fault zone shale/clay content using simple, field calibrated algorithms. For example, the Shale Gouge Ratio (SGR) algorithm (Yielding et al. 1997) is a measure of the amount of shale that passes a point on a fault, which is considered proportional to the amount of shale that can be incorporated into the fault rock by mechanical processes (Figures 7 and 8).

Figure 8. For a given interval, Vshale is the volumetric shale fraction and ∆Z the interval thickness.

1. Seismic interpretation

250m

100m

2. Fault surface map

3. Across-fault juxtaposition map

Low

High 4. Fault seal potential

Fault surface contoured for displacement

Throw

dZ1

dZ3

dZ4 dZ5

dZ2 SGR = ∑ (Vshale • ∆Z)

Throw

Vshale1

Vshale3

Vshale4 Vshale5

Vshale2

15

SGR values, or equivalent, can be converted to permeability by calibrating against field (pressure) data or compared against calibrated databases (see Bretan et al. 2003). Sequence shale content and thickness are extracted from well log data and/or borehole samples, whilst throw is typically extracted from seismic data. An example of the use of this type of approach to assess the impact on groundwater flow is presented in Bense and Van Balen (2004). Seismic Data Figure 9 shows the positions of onshore 2-D seismic lines and interpreted faults in the study area. Determination of throw (vertical component of displacement) from seismic data is by far the main concern for a fault seal appraisal and this is the focus of the following assessment. The seismic lines in Table 2 were selected from the total dataset for appraisal because they traverse the main fault zones of interest. Most lines were accessed through GeoScience Victoria (Minerals and Petroleum Division, Department of Primary Industries). It was decided that the data are of insufficient quality to merit loading onto CSIRO Petroleum’s seismic workstations. Seismic resolution is variable, but, at best, is estimated to be in the order of 20-30 m. That is, faults with vertical displacements (throw) less than 20-30 m are not imaged. This is a severe limitation, given that faults of this size are expected to be in the order of 1-15 km in length, based on known measured lateral displacement gradients of normal faults (0.02-0.002; Walsh and Watterson, 1989). This, combined with additional complexities discussed below hinders interpretation of faults and their displacements. Nevertheless, large faults are discernible on the seismic data. Figure 10 highlights some of the large basin-forming faults that are thought to be important to fluid flow, and are potentially resolvable on the seismic data. A number of seismic lines that cross these fault zones was examined in detail, examples which are shown in Figures 11 and 12.

16

Figure 9. Map of the study area showing distributions of onshore 2-D seismic lines and oil (green) and gas (red) fields.

Figure 10. Map of the study area showing distributions of wells and traces (dashed lines) of the main faults taken from Figure 9 and qualified using the seismic lines annotated. Arrows point to downthrown side (with approximate throw in milliseconds (ms) where discernible).

Seaspray Depression

Rosedale Fault

Darriman Fault

200ms

250ms 65ms

17

Table 2. Seismic lines selected for analysis that traverse main faults. ‘IESX’ is a Schlumberger seismic interpretation package and indicates that these lines are held on the Department of Primary Industries system (SEGY data also available). SEGY is a format for storage of seismic data. Paper indicates that analogue copies only are available.

LINE Format Darriman Fault

Rosedale Fault

Supplied by DPI Description

GWS65BL-III Paper Very poor resolution GWS65B-12 Paper Very poor resolution GGS185AE-02

? Reverse fault & inverted normal fault

GCR87A-2 SEGY Low amplitude fold – no faults imaged

GCR87A-3 SEGY As above GCR87A-22 SEGY As above GCR87B-107 IESX Fault zones clearly imaged GCR87B-109 SEGY X GCR87B-105 SEGY X GCR87B-103 SEGY X GCR87B-101 IESX X GCR87A-1 SEGY X GAW62A-AI Paper X GWS65B-04 Paper Very poor resolution GWS69A-52 Paper Poor GWS69A-61 Paper Poor GB82A-05 SEGY No notable structure imaged GB82A-6A SEGY No notable structure imaged GB82A-09 SEGY X GB82A-10 SEGY No faults imaged GHG85A-2 Paper OK – large faults with folds

imaged GHG85A-5 Paper OK – large faults with folds

imaged GHG85A-6 Paper As above GHG85A-9 Paper Fold/fault? GCRP91A-05 IESX Strike line – poor orientation to

image faults GCRP91A-06 IESX OK – large fault zone/fold

imaged GCRP91A-07 IESX Only low amplitude folds

imaged GCRP91A-07 SEGY ,, GCRP91A-11 OK – complex inverted fault

zone GSG91A-03 IESX X GMPV97-01 IESX OK – complex inverted fault

zone GMPV97-01 IESX OK – complex inverted fault

zone

18

Figure 11. Seismic line GGSI85AE-02 through the Darriman Fault. Note normal offset gives way upwards to a fold.

Figure 12. Seismic line GCR87B-107 through the Darriman Fault showing large normal offset.

19

The majority of faults observed on the seismic lines display normal offsets, but are frequently associated with folds, notably in their hanging walls and above the faults themselves (Figures 11 and 12). The geometry of these faults and associated folds is strongly suggestive of the compressional reactivation (inversion) of an extensional normal fault system, which is supported by minor reverse offsets on some faults (right fault; Figure 11). This chronology conforms to the previous tectonic assessments of the Gippsland Basin, documenting Jurassic-Early Cretaceous extension and subsidence into the Eocene followed by Tertiary inversion (see Willcox et al. 1992 and references therein). The resultant structural geometries pose the significant problem for a fault seal analysis.

1) Folds are clearly imaged on the seismic lines, but due to the poor seismic resolution of some lines, it is uncertain if they are associated with faults;

2) The faults have accommodated at least two main, and significantly contrasting, phases of displacement (normal followed by reverse), which has the result of throws:

a. Locally reversing, and; b. Varying significantly laterally (e.g. about 250m throw to the

south (Figure 12) changing to about 65m throw to the north (Figure 11) over approximately 20km).

The consequence is that correlating faults between km-spaced seismic lines is problematic. For example, the Darriman Fault identified on Figures 11 and 12 may not actually be the same fault. Furthermore, poor resolution and variable displacement patterns make it unlikely that critical features such as relay zones (up to km-scale map dimensions; see Bense and Van Balen, 2004), which may focus flow, could be identified. Shale gouge/clay smear calculations are heavily dependent on fault throw and therefore poor throw determinations make fault seal calculations in this environment too error prone. For example, initially large displacement normal faults that displace the aquifer past a large cumulative volume of shale prone units will possess a high seal potential. However, subsequent to inversion, the cumulative normal displacement will be reduced or even reversed. SGR calculated using the finite displacements in this environment will therefore be compromised. On a positive note, despite the poor seismic resolution of some lines, the main faults are clearly imaged on the better lines, demonstrating the onshore expression of both the Darriman and Rosedale fault systems. Onshore Fault Seal Summary The onshore fault analysis confirmed the location of onshore extensions of large basin forming fault zones, however, using publicly available data, a meaningful investigation of the role of faults as barriers and seals to groundwater flow from standard fault-seal analysis techniques is not possible. This is due to the insufficient quality of the seismic data to define the position and displacements along the major faults. Tectonism in this

20

region has resulted in pre-existing normal faults being reactivated in compression, meaning that it is not possible to elucidate the magnitude of displacements for both extensional and contractional events. It is anticipated that the Latrobe aquifers are both completely offset (juxtaposition seals) and self-juxtaposed (requiring the fault rocks to provide the seal) along the major fault zones due to the local reversal of finite displacements subsequent to the reactivation event. Therefore, we must rely on observed across fault pressure discontinuities to indicate the relative effectiveness of the fault zones as either fluid conduits or barriers.

21

COASTAL SUBSIDENCE RISK Concerns of possible coastal land subsidence due to pressure decline in the Latrobe Aquifer have been discussed by Hatton et al. (2004). SKM (2001) modelled subsidence at Yarram and concluded that there was a 10% chance that subsidence will exceed 800 mm. This was based on a projected cessation of oil and gas production by 2023 with some surface rebound expected to follow post 2023 termination of offshore production. These calculations have been conducted by SKM (2001) using Terzaghi’s theory of one- dimensional consolidation (e.g. Scott, 1980). Hatton et al. (2004) recommend that a more comprehensive risk assessment study of potential subsidence in the coastal region should include petrophysical measurements and further subsidence modelling. Both Hatton et al. (2004) and SKM (2001) identify the lack of measured physical rock parameters for the Latrobe Aquifer as contributing the most uncertainty to numerical simulation results. Furthermore, the subsidence risk is geographically variable along the Gippsland coast due to variations in: 1) the degree of water level decline (hydraulic head) in the Latrobe Aquifer System; 2) coastal topography; and, 3) the composition of the Latrobe Aquifer rock lithology and its associated rock properties. Objectives This study aims to quantify and assesses the risk of subsidence at specific locations along the Gippsland coast. With the knowledge of the coastal topography and incorporation of climate change modelling (sea level variations and 100 yr storm wave heights forecast), the subsidence model results can be linked in a GIS and the relative impact of subsidence along coastal areas can be evaluated. The results can drive strategies to mitigate areas of high subsidence risk. This will enable the future incorporation of observational data from the high resolution GPS monitoring sites operated by Victoria DPI. For prediction of subsidence, reliable rock properties are needed. Since the cost of obtaining measured rock property data is high, this can only be done for a limited number of well locations. In order that samples are obtained from the most critical locations a staged approach is taken, where:

1. The analytical solution for one-dimensional consolidation is implemented for calculating the degree of consolidation with time assuming the exact mean conditions as modelled by SKM (2001) so that the results of the new modelling can be compared with SKM (2001) simulation results.

2. A FLAC3D (Itasca, 2002) one-dimensional ultimate consolidation model is constructed and run with parameters identical to those of the SKM (2001) model so that model results can be compared.

3. A sensitivity analysis is conducted on all model parameters that are unconstrained for the Gippsland coastal study area to identify which lead to the most uncertainty in the model results. The most critical parameters can then be targeted when, in the future, measured data is obtained.

22

4. The FLAC3D (Itasca, 2002) one-dimensional consolidation model is run for a coastal location in the Yarram area based on the known stratigraphy, hydrodynamic characterisation (Hatton et al., 2004), and a range of values for unconstrained parameters.

5. Extension of the one-dimensional model to three-D and estimation of the relative risk of subsidence along the Gippsland Coast due to various abstraction activities incorporating coastal topography, hydrodynamic characteristics, forecast storm wave heights, and a range of values for unconstrained parameters.

6. A sampling program of the most important rock properties (from the sensitivity analysis) in the regions of highest risk is designed based on 5.

7. Refinement of the numerical modelling with the added constrains from the stage 6 sampling program.

This report forms the results of the first three stages. The mean parameters used by SKM and the model assumptions are implemented. We perform the calculations using the analytical solution for one-D consolidation with time integration conducted in an Excel spreadsheet and compare the results with those of SKM (2001). We conduct numerical simulations using ITASCA software FLAC3D and conduct a sensitivity analysis on the main unconstrained model parameters.

Consolidation Theory

The framework of sediments and soils consist of a compressible skeletal fabric and pore fluid. The mineral grains themselves and the pore fluid are considered incompressible. A load (σ) applied to a volume element of sediments is carried partly by the pore fluid phase (P) and partly by the solid skeleton (σ`). In a sample of saturated sediments where fluid flow is not allowed the pore pressure increases immediately upon loading. If the pore fluid is gradually allowed to escape, the pore pressure dissipates (at a rate that depends on the permeability) and the load gets transmitted to the solid skeleton (Figure 13a). In response to this stress the solid grains begin to rearrange to occupy a smaller volume (the porosity decreases). This stress that causes the volume change is called the effective stress (σ`=σ−P) as it is effective in causing the compression. The settlement in the vertical direction is what is called subsidence. The amount of settlement is dependent on the sediment compressibility which is the volume change per unit volume due to a unit increase in effective stress.

Processes that cause the effective stress increase include the natural process of deposition; the placement of various structures on top of sediments (Figure 13a) and pressure drop due to abstraction at constant overburden pressure (Figure13b). Rebound on the other hand is the reverse process and can take place when the effective stress is reduced by processes like erosion, tectonic forces and over pressurizing.

Figure 14 is a typical consolidation diagram; it shows the reduction of sediment volume expressed in the reduction of the void ratio or pore space as the effective stress increases. A normally consolidated material has never before been subjected to an effective stress higher than the

23

current stress. This is what happens during deposition of new sediments or abstraction of pore fluid for the first time. The material, as a result, would have higher compressibility (experiences higher deformation when subjected to a specified load) (as shown by the line A to C). On the other hand, a material, which has been unloaded from stress level B to lower stress level D, and is currently being loaded on the stress path DB, is called an overconsolidated material. This would have a lower compressibility (experiences less deformation when subjected to the same specified load because it has been hardened) than AC. Such a situation arises when overlaying sediments are eroded away (i.e. reducing the load), under the effect of certain conditions of changing tectonic force, or conditions of increasing pore pressure. The highest effective stress a material has been subjected to is called the preconsolidation stress. The value of the effective stress at point B for the sample undergoing unloading and loading on the path BD is its preconsolidation stress σ'c0.

Sudden load increment = ∆σ

Pressure dissipates slowly with time and effective stress increases with time

∆σ

σ

σ=total stressσ`=effective stressP=pore pressure

Immediate increase in pore pressure = ∆σ Low. perm.Due to

subsidence

Subsidence = consolidation due to mechanical loading

0Time after Loading

Time

P0Pt

σ`0

P0

σ`t

Effective stress profile=What the sediments feel

Total stress profile

Stre

ss/P

ress

ure

Pore pressurepo

rosi

ty

σ`se

ttlem

ent

Figure 13a. One dimensional consolidation: A step load causes spontaneous increase in pore pressure that dissipates with time depending on the permeability.

24

Total load stays constant

Effective stress increases with pore pressure decrease with time

σ =total stressσ`=effective stressP =pore pressureHead loss=P0-PtPf = pore pressure end of abstraction, start of recovery

pore pressure decreases with abstraction with time

subsidence

Abstraction and Recovery

abstraction

poro

sity

σ`

settl

emen

t

σ

0Time after abstraction start

Time

PfPt

σ`0

P0 σ`tEffective stress profile=What the sediments feel

Total stress profileS

tress

/Pre

ssur

ePore pressureduring abstraction

Pore pressureduring recovery

recovery

porositysettlement

while

Pore pressureAfter full recovery

Figure 13b. One dimensional consolidation: Abstraction of pore fluid increases effective stresses and results in settlement-We assume total stress is constant.

Normal consolidationcurve during deposition,

ice surcharge or fluidabstraction

Effective stress ( )��

CVf001-06

Vo

idra

tio

(e)

A

B

C

D

e0

��0 ��01

e01

��C0

recompressioncurve

Swelling curveduring erosion or

ice melting

Figure 14. One-dimensional consolidation: a void ratio vs. effective stress plot of typical sediments.

25

Terzaghi (1943) suggested a model for the one-dimensional consolidation of a soil sample in an oedometer test that gives the degree of consolidation with time in response to a step change in the load. It is based on coupling the stress strain relationship with the continuity equation for fluid flow. The deformation is assumed to be a result of a step increase in the stress at the top of a thin compressible layer which is constrained in the horizontal direction. In this case, the immediate pore pressure change in the soil is assumed to be uniform and the fluid flow (dewatering) is assumed to take place only in the vertical direction. The soil and water particles as mentioned above are assumed to be incompressible and the soil remains saturated so that the volume change caused by the load application is only due to the expulsion of water. The formation modulus of volume compressibility is assumed constant as well as the formation permeability. This model produces a differential equation that can be expressed either in terms of the excess pore pressure or the resulting effective stress change. Analytical solutions exist for this equation for the case of uniform or triangular distribution of initial excess pore pressure. The case in the oedometer test and the case depicted in Figure 15a are similar as they both have uniform initial excess pore pressure distribution across the depth of the layer. We use this same model to simulate the effect of abstraction. Pressure loss generates equal effective stress increase Figure 15b. The average degree/rate of consolidation (U), which assumes a value of 100 upon completion of consolidation (full dissipation of excess pore pressure), is expressed as follows (Scott, 1980).

⎟⎠

⎞⎜⎝

⎛−−= ∑

∞

=0

22 )exp(21100

mvTM

MU (1)

where ( ) etc.,...2,1,0,)122

=+= mmM π

An approximate expression for the average degree of consolidation (eqn. 1) can be found in Whitlow (1995)

60%U for 1004

2

<⎟⎠

⎞⎜⎝

⎛=UTv

π , or (2)

60%U for )100log(933.0781.1 ≥−−= UTv (3)

where Tv is a nondimensional time factor defined as.

2dtCT v

v = (4)

26

(a) (b) Figure 15. a) A uniform load applied at the surface produces a uniform effective stress increase in the thin compressible layer. b) Pressure loss due to abstraction generates equal effective stress increase. where t is the real time, d is the drainage path (equals layer thickness for a single drainage layer and half of that for a double drainage layer), and Cv is the consolidation index which is the ratio of the hydraulic conductivity and the compressibility of the sediment normalized by the unit weight of water. Methodology In the following section we use the analytical solution above to calculate subsidence or rebound due to effective stress increase or decrease at the compressible sediments that are experiencing pressure decrease or increase in the aquifer. Afterwards we develop a preliminary FLAC3D one-dimensional consolidation model to calculate the ultimate subsidence and demonstrate settlement sensitivity to input parameters. This numerical model does not include transient flow effects and hence the settlement produced by the model is the ultimate settlement (the immediate settlement of a highly permeable formation). This model will later be extended to a three dimensional model and will include transient flow effects. A useful relation that we will need prior to proceeding to the numerical modelling section is the relation between the inverse compressibility: the constrained stiffness modulus Em (usually obtained from a confined compression test, a loading case similar to one-dimensional consolidation case), and the stiffness modulus E of the sediments (usually obtained from a triaxial test where the deformation is not restricted to one dimension). This relation is as follows.

( )( ) ( )νν

ν211

1−+

−=

EEm (5)

27

where ν is Poisson’s ratio, which controls the lateral deformation in an unconfined compression test due to axial loading. Moreover it is worth noting that the elastic (Sske) and plastic (Sskv) coefficients of the skeletal specific storage are defined as follows.

mv

wskv

me

wske

ES

ES

γ

γ

=

=

(6)

where γw is the unit weight of water, Eme is the constrained elastic stiffness modulus (equals inverse of the elastic modulus of volume compressibility) and Emv is the constrained plastic stiffness modulus of the formation. Where the term elastic refers to recoverable mechanical behaviour and the term plastic refers to the non recoverable behaviour.

When the sediments are consolidated from the effective stress σ’0 to the effective stress σ’

fin, and considering the preconsolidation stress σ’c0 then the ultimate settlement (∆H) is calculated as follows.

( ) ''0

'0

'0 0

;/cfinwskvSHH σσσσγ ≥−=∆ (7)

( ) '''0

'0 0

;/cfinfinwskeSHH σσσσγ <−=∆ (8)

( ) ( ) '0

''0

''0

'00 :// ][ cfincfinwskvcwske SSHH σσσσγσσγ >−+−=∆ (9)

where H0 is the initial sediment thickness. Other parameters are as defined above. Equation (7) is the case of normally consolidated sediments, equation (8) applies to overconsolidated sediments and equation (9) applies to sediments starting as overconsolidated and becoming normally consolidated due to being subjected to high stresses. Model Assumptions We assume that the change (excess or reduction) in pore pressure ultimately produces a uniform effective stress across the thickness of each aquifer (Balook and Latrobe) as in an oedometer test. We follow Helm’s (1976 and 1984) assumption that in confined aquifers the effective stress change equals the pore pressure change as the total stress remains constant and in agreement with the principle of effective stress. The pressure change can be modelled as a stress change at the aquitard boundary, Helm (1976). This assumption justifies the use of Terzaghi’s analytical solution in equation 1 to obtain the average settlement with time. Linear abstraction and recovery rates (to match SKM 2001) are approximated by a series of step loads of values that vary with time. Transient effects of non-vertical fluid flow and any non-vertical drainage due to abstraction or recovery are neglected and only vertical settlement is considered as the aquitards (the compressible impermeable layers inter bedded in the aquifers) are thin and extensive in the horizontal direction. In the case described by SKM report,

28

Sinclair (2001) the rates of head loss in each of the two aquifers are different, despite the fact that they have the same permeability and are hydraulically connected. Figures 16(a-b) are reproduced from the SKM report and represent the pressure head of the Balook and Latrobe formations during abstraction and various recovery scenarios. Following SKM assumptions we use the parameters in Table 3. When the sediments are normally consolidated the stress strain relations are controlled by the coefficient of plastic specific storage Sskv and when the sediments are over consolidated, the stress strain relations are controlled by the coefficient of elastic specific storage Sske. In scenarios of recovery, a pressure head loss of 4 and 5 m is maintained for the Balook and the Latrobe sequences respectively. Figure 16c shows the thicknesses of the Balook and Latrobe sequences and the thickness of the overlying layer. Although Sinclair (2001) refers to the majority of the upper layer as “compressible Alberton Coal Measures”, it is not contributing to subsidence in the modelling as it doesn’t experience pressure loss. The head for each layer, and the sea level, are shown in Figure 16c, where the datum is the base of the Latrobe formation. Figure 16d shows the total head, elevation head and the pressure head for both compressible sequences before abstraction commencement and after the cessation of abstraction.

Table 3. Model Parameters.

Parameter Balook Formation Latrobe Formation Depth below ground level (m) 143 231 Compressible formation thickness in simulation (m)

88 129

Coefficient of plastic specific storage Sskv m-1)

2x10-4 2x10-4

Coefficient of elastic specific storage Sske (m-1)

2x10-5 2x10-5

Permeability (myr-1) 4x10-4 4x10-4 Preconsolidation stress (m) 100 190 Head in 1969 (mAHD) 29 42 Minimum head at end of abstraction (MAHD)

0.5 -11.2

Drawdown (head loss) rate (myr-1) 0.54 1.02 Drawdown duration (yrs) 53.5 53.5 Net lost head in case of recovery (m) 4 5 Recovery periods modelled Infinite, 5yrs or 30 yrs Infinite, 5yrs or 30 yrs

29

Figure 16a. Water head and abstraction and recovery rates for the Balook Formation reproduced from the SKM report (Sinclair, 2001).

Figure 16b. Water head and abstraction and recovery rates for the Latrobe formation reproduced from the SKM report (Sinclair, 2001).

30

SKM model: Alberton East No 3

129.0

217.0

360.0

Balook

Latrobe129

88

143

Sea Level8.7 m Ground Level

393.3Total Initial Head Latrobe =42m Balook =29m

Z

0.0

Figure 16c. The compressible sequences are the Balook and the Latrobe formations. The initial head indicated, (Sinclair, 2001).

0

50

100

150

200

250

300

350

400

450

0 50 100 150 200 250

Elevation (m)

Hea

d (m

)

elevation_head

pp_head

Total_head

pp_head at end ofabstraction

Total_head at end ofabstraction

Figure 16d. Total head, pressure and elevation head before and after cessation of abstraction for the Balook and Latrobe formations.

The Analytical Solution Results We implement the analytical solution for Terzaghi’s one-dimensional consolidation model for the case of uniform initial excess pore pressure distribution, and calculate the average degree of consolidation with time (equations 2 and 3), in an Excel worksheet. The constant effective

31

stress on the doubly draining formation boundaries is ramped up or down with time, depending on the amount of reduction or increase in pore pressure at stages as abstraction, or recovery, takes place. Each stage of pore pressure change produces an equal change with reverse sign in the effective stress (constant total stress). The subsidence resulting from each load increment is calculated at each time step (calculation stage) and its effect is accumulated with time. The total subsidence for a formation at a certain time, results from adding the contribution of all abstraction stages so far (at all the previous stages). The final subsidence sensed at the ground level is due to settlement of both compressible sequences (Balook and Latrobe) and is obtained by summing up their contributions. Figure 17 shows the resulting subsidence for the three scenarios of recovery modelled: No recovery; 5 years recovery; and, 30 years recovery. The SKM model subsidence results for 5 and 30 years recovery scenarios are shown in Figures 17d-e. Our results are based on preconsolidation stresses of 100 m and 190 m water equivalent for the Balook and the Latrobe formations. The formations remain elastic (overconsolidated) during the abstraction time. The values calculated for the initial effective stresses, the final effective stresses after 53.5 years of abstraction and the preconsolidation stresses are shown in Figure 18. Although the resulting average subsidence with time is of the same order of magnitude as in the SKM model, they are not identical. As an example in our model, the no recovery case produced 137 mm subsidence after 100 years from beginning of abstraction while the SKM deterministic model produced about 304 mm subsidence, summary of results are shown in Table 4. Our understanding and calculation show that the ultimate settlement for these overconsolidated sediments (when subsidence is 100% complete), should not exceed 192 mm. Our solution shows the occurrence of substantial subsidence after cessation of abstraction in the no recovery scenario indicating continuing compaction of the layers while the SKM model shows only 15 mm subsidence for the same case. The ultimate settlement of 192.0 mm was not yet reached in our model after 100 years from commencement of abstraction due to the formation low permeability (small time factor in equation 5) leading to the delay in the effective stress increase across the compacting sequences.

32

Total subsidence Balook and LatrobeTime (yrs)

-160

-140

-120

-100

-80

-60

-40

-20

0

1969 1989 2009 2029 2049 2069

Time (Yrs)

Subs

iden

ce (m

m)


-90-80-70-60-50-40-30-20-10

0

1969 1989 2009 2029 2049 2069

Time (Yrs)

Subs

iden

ce (m

m)

Figure 17a. Total average subsidence (mm) vs. time (yrs). No recovery scenario.

Figure 17b: Total average subsidence (mm) vs. time (yrs). 5 years recovery.


-100-90-80-70-60-50-40-30-20-10

0

1969 1989 2009 2029 2049 2069

Time (yrs)

Subs

iden

ce (m

m)

Figure 17c: Total average subsidence (mm) vs. time (yrs). 30 years recovery.

Figure 17d. Subsidence (mm) vs. time (yrs) as predicted by SKM model for 5 yrs recovery, (Sinclair, 2001).

Figure17e. Subsidence (mm) vs. time (yrs) as predicted by SKM for 30 years recovery, (Sinclair, 2001).

Figure 17. Total subsidence of formations considering preconsolidation stresses of 100 m and 190 m water equivalent for the Balook and the Latrobe formations.

33

Preconsolidation stress

0.00E+00

1.00E+06

2.00E+06

3.00E+06

4.00E+06

5.00E+06

6.00E+06

0 50 100 150 200 250

Depth from base of Latrobe

Str

ess

(MP

a)

Preconsolidation stressInitial effective stress 1969Final effective stress 2023

Figure 18. Effective stress change at end of abstraction, comparison to preconsolidation stress

Table 4. Subsidence results in mm for all cases studied, comparison with SKM model results.

Scenario Our results using the analytical model

SKM model deterministic or 50% values

Year 2022 Year 2069 Year 2022 Year 2069 No recovery scenario 82 137 289 304 5 years recover 82 47.3 289 >40 30 years recovery 82 56.1 289 >40

34

The Numerical Model In the current preliminary numerical model we simulate the ultimate one dimensional settlement of a column of saturated sediments representing the Balook and Latrobe formations due to abstraction and various scenarios of recovery. The model is initially equilibrated under its own weight. We assume the top layer to be fully saturated. The initial condition for the model (zero subsidence) corresponds to the state where the total head for the Balook formation was 29m and that for the Latrobe formation was 42m. The top boundary of the Balook formation is subjected to a constant total stress. Abstraction/recovery is modelled by gradually reducing/increasing the pore pressure in each formation uniformly (uniform pressure change across the thickness of each layer) at small increments (time steps). The effective stresses are then increased or decreased to account for loss or gain of pore pressure. The corresponding subsidence or swelling is then calculated. This continues and the subsidence/swelling is accumulated until the modelled abstraction and recovery period is over. Time consumed to equilibrate pore pressure and effective stresses changes are not included in the model as it does not have fluid flow simulation (not even vertical), but rather deals with the net result of the abstraction or recovery.

Modelling Stages and Input Parameters

The following detailed stages were conducted in the simulation process. Preconsolidation stage

1. Load the mesh with soil dry weight. 2. Set the displacements to zero. 3. Saturate the formation with water. 4. Set the displacements to zero. 5. Sink the model under water (consider total head) + dry weight of

overlying formation 6. Set the displacements to zero.

Abstraction/recovery stage 1. Abstraction for 53.5 years 2. Recovery Scenarios: no recovery, 5 years recovery, and 30 years

recovery.

The model is elastic if it is overconsolidated or plastic if it is normally consolidated. This depends on preconsolidation stress σ'c0

The models geometrical dimensions and mechanical properties are defined in Table 3.

The analytical solution uses the constrained modulus (inverse of Skse/ γw) (from equation 6) as an input while the numerical model requires Poisson’s ratio and the modulus of elasticity. The model is constrained horizontally by imposing zero horizontal displacement on the vertical boundaries. The relation between the constrained and the non-constrained moduli was shown in equation 5.

In the first part of the simulations, we use Poisson’s ratio ν=0.25 and select E to match the Em used by SKM report. In the second part of the simulations, we demonstrate the impact of the preconsolidation stress,

35

by assigning a small value to the preconsolidation stress, which implies that the sediments are on the normally consolidated path through out the abstraction process.

The third set of simulations tests the effect of varying the sediment compressibility on the subsidence. We vary Poisson’s ratio and hold the stiffness elastic modulus constant. This is equivalent to testing the effect of the selected constrained modulus or the Skse value proposed by SKM and used in the analytical model.

Numerical Simulation Results for Ultimate Subsidence using SKM Parameters

Figure 19 shows simulation results of the ultimate subsidence using SKM parameters and Poisson’s ratio of 0.25. We model abstraction followed by 5 years of recovery. Figure 19a shows the ultimate subsidence/swelling with time. The contour lines show the displacement at the end of recovery. The total vertical stress and pore pressure contour lines at the end of the simulation are shown in Figures 19b and c. Figure 19b shows also 2 lines, the upper horizontal line shows that the total stress at the top zone remains constant with time while the initially increasing (corresponds to abstraction) and then decreasing (corresponds to recovery) line is the effective stress. The pore pressure change, as the line in Figure 19c shows, causes the effective stress in Figure 19b to change resulting in subsidence, Figure 19a. The subsidence shown, 192mm, is the ultimate elastic compaction.

Figure 19a. Displacement contours and vectors at end of abstraction and 5 years recovery. The line represents subsidence (mm) vs. time (yrs) during abstraction and recovery.

36

Figure 19b. Contours of the total vertical stress (MPa) at the end of abstraction and 5 years recovery. The upper line is the total stress and the lower line is the effective stress (MPa) at top zone during the abstraction and recovery simulation steps.

Figure 19c. Pressure contours (MPa) at end of abstraction and 5 years recovery and pressure value (MPa) vs. time (yrs) at top zone during abstraction and 5 years recovery.

37

The 30 years recovery scenario is presented in Figures 20. The line in Figure 20a shows the ultimate subsidence and swelling with time. The contour lines show the ultimate settlement at the end of recovery. The total vertical stress and pore pressure contour lines at the end of the simulation are shown in Figures 20b and c. Figure 20b shows 2 lines, the upper horizontal line shows that the total stress at the top zone remains constant with time while the increasing and then decreasing line is the effective stress. The pore pressure change, as the line in Figure 20c shows, causes the effective stress to change, as shown in Figure 20b, resulting in subsidence as Figure 20a shows.

Figure 20a. Displacement contours and vectors at end of abstraction and 30 years recovery. The line represents subsidence (mm) vs. time (yrs) during abstraction and recovery.

38

Figure 20b. Contours of the total vertical stress (MPa) at end of abstraction and 30 years recovery. Upper line is total stress and lower line is effective stress (MPa) at top zone during the abstraction and recovery simulation steps.

Figure 20c. Pressure contours (MPa) at end of abstraction and 30 years recovery and pressure value (MPa) vs. time (yrs) at top zone during abstraction and 30 years recovery

39

Numerical Simulation Results for Ultimate Subsidence: Sensitivity to Preconsolidation Stress There has been an ambiguity in the expressed values of the preconsolidation stress in the SKM model. This can be a reason contributing to the differences in results between SKM model and our analytical solution results. This section demonstrates the effect of the preconsolidation stress. Two values for preconsolidation stresses have been investigated. Preconsolidation stresses of 1.172 MPa and 2.23MPa for the Balook and the Latrobe formations correspond to the case of normally consolidated sediments and preconsolidation stress of 10MPa for both formations corresponds to the case of over consolidated sediments (Figure 14). When the sediments are normally consolidated we follow the assumption that they are 10 times more compressible as per the SKM model parameters (see Table 3, Sske=1.67x10-5 m-1, and Sskv=1.67x10-4 m-1). The subsidence predicted for normally consolidated sediments was 1.92 meters compared to 0.192 m for the overconsolidated sediments (Figure 21). In the normally consolidated case, only 0.16 m of the subsidence was recoverable after 5 years recovery.

Figure 21a. Subsidence (mm) vs. time (yrs) for normally consolidated sediments for 5 years recovery case.

40

Figure 21b. Subsidence (mm) vs. time (yrs) for over consolidated sediments for 5 years recovery case. Sensitivity to Sediment Compressibility The current numerical model simulates one-dimensional ultimate compaction and will be extended to three-D. A necessary input in the numerical simulations is Poisson’s ratio. Four simulations were conducted to present the effect of Poisson’s ratio (Figures 22) on subsidence while keeping the modulus of elasticity constant at 5x108 Pa (refer to equation 5). This is equivalent to varying the coefficient of elastic skeletal specific storage Sske, (eqn. 6). Ultimate subsidence values range between 161.2 and 51 mm as Poisson’s ratio varies from 0.25 to 0.45 and the Sske varies from 1.67x10-5 to 0.53x10-5 m-1. Figures 23 and Table 5 show that the subsidence was decreasing with increasing Poisson’s ratio and increasing with increasing elastic skeletal specific storage Sske. Therefore it is very important to constrain the sediment compressibility used in the simulations.

41

Figure 22a. Subsidence (mm) with time (yrs) for E=5x108 Pa, Poisson’s ratio=0.25 and 5 years recovery case.

Figure 22b. Subsidence (mm) with time (yrs) for E=5x108 Pa and Poisson’s ratio=0.3 and 5 years recovery case.

42

Figure 22c. Subsidence (mm) with time (yrs) for E=5x108 Pa and Poisson’s ratio=0.35 and 5 years recovery case.

Figure 22d. Subsidence (mm) with time (yrs) for E=5x108 Pa and Poisson’s ratio=0.45 and 5 years recovery case.

43

Figure 23a. Variation of subsidence (mm) with the elastic skeletal specific storage Sske (m-1).

Figure 23b. Variation of subsidence (mm) with Poisson’s ratio.

Table 5. Summary of the influence of the mechanical parameters Sske

and ν on the ultimate subsidence.

Sske (m-1) X10-5

ν corresponding to E=5x108 Ultimate subsidence (mm)

1.67 0.25 161.6 1.49 0.3 145.5 1.25 0.35 120.7 0.53 0.45 51

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Discussion The degree of subsidence due to pressure depletion is dependent on the time, the compressible layer thickness, the drainage path and the hydraulic conductivity of the layer (equations 1, 2, 3 and 4). The final settlement at completion of consolidation is dependent on the effective stress increment, the layer thickness and its modulus of volume compressibility. It is also dependent on the preconsolidation stress which determines the compressibility and hence the amount of irrecoverable settlement. The sediments compressibility and hydraulic conductivity vary with the depth and stress level (e.g., Helm 1976, 1984). The compressibility and the hydraulic conductivity are like the porosity, decrease with depth. The preconsolidation stress depends on the loading and erosion history of the compacting layer. Accurate determination of these mechanical and hydraulic parameters is necessary for correct prediction of subsidence. An oedometer laboratory test (e.g. Scot, 1980) performed on a sample from the sediments can provide the consolidation parameters including the hydraulic conductivity. For three-dimensional modelling, knowledge of Poisson’s ratio and the non constrained stiffness modulus is necessary and can be obtained from a triaxial test. Moreover, there are typical values for some consolidation parameters (e.g., Budhu, 1999) and some useful empirical relationships in the published literature. These include relationships between consolidation parameters and parameters that can be measured easily like the liquid limit, the void ratio and the water content (e.g. Azzoz, 1976). The hydraulic conductivity can also be measured in the laboratory using either a constant or a falling head permeameter tests, depending on the sediments expected permeability. In the case of non availability of test samples, the hydraulic conductivity can be obtained from costly in situ measurements: constant head in situ permeability test for the clay layers or in situ pumping tests for the aquifer material. The approach we have used in implementing the analytical solution to calculate subsidence assumes that a reduction in pressure head in the aquifer imposes an equal increase in the effective stress at the boundary of the aquitard. The SKM report (Sinclair, 2001) does not distinguish between the aquifer and aquitard and effective properties are assigned to the aquifer and the inter bedded compressible layers. We have imposed an effective stress equal to the measured pressure head drop (0.54 and 1.02 m/year respectively) on the upper boundaries of the Balook and the Latrobe sequences shown in Figure 17a and calculated the subsidence due to the resulting internal effective stress change with time. This is done using an analytical solution for one-dimensional consolidation (equations 2 and 3) that calculates the average consolidation with time for the compressible sequences in Alberton East 3 in the Gippsland basin. Results obtained for the various scenarios of recovery are not in agreement with the results of the SKM subsidence model as table 4 shows. This could be due to various reasons, one of them is that based on the SKM report, we assume the formations to be overconsolidated. If part of the formation becomes normally consolidated towards the end of the loading stage this can increase the

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consolidation to a large value. This can also increase the ultimate settlement value as the last two rows in table 6 show. The current preliminary numerical model is implemented to impose the pore pressure change on the model and to calculate the ultimate subsidence. The numerical model is used to conduct a parametric study of the controlling parameters of the problem. The analysis conducted shows the importance of the preconsolidation stress value as its impact on the predicted subsidence can be an order of magnitude different as the second and third rows of table 6 show. Model results are very sensitive to the input mechanical compressibility parameters, Poisson’s ratio. Analysis has shown that subsidence can increase by three fold when Poisson’s ratio drops from 0.45 to 0.25. This numerical tool can be developed to test scenarios other than one-dimensional consolidation, and to model ground water flow and the exact settlement in three-D for the next stage of the subsidence modelling study.

Table 6. Effect of preconsolidation pressure

Case Subsidence (mm)

Analytical solution at year 2069 –over consolidated with no recovery

137

Numerical model (ultimate settlement) -over consolidated at 2069 with no recovery

192

Numerical model (ultimate settlement) -normally consolidated at 2069 with no recovery

1920

Subsidence Modelling Conclusions and Recommendations Land subsidence is the time dependent settlement of sediments resulting from the expulsion of water. The subsidence modelling conducted in this study shows a maximum subsidence of 137 mm at the location of the Alberton East No 3 bore with the projected cessation of abstraction by year 2023 and considering no pressure recovery. This result is obtained using parameters of high uncertainty with the exception of the stratigraphy. The importance of constraining the subsidence parameters of the formation on its response to pressure drop has been demonstrated. As we have shown, a small variation in the compressibility parameters has a large impact on the formation subsidence. The loading history of the formation, including erosion and deposition episodes and fluid drawdown and recovery events, is important. It is what determines the severity of the subsidence and whether the formation will act elastically or plastically. Sediments which have never experienced vertical effective stresses greater than the current vertical stress are expected to subside orders of magnitude greater than sediments which have hardened due to experiencing greater effective stresses than their current effective stress. Moreover only part of this subsidence can be recovered upon restoring the lost pressure head in the first type of sediments. Based on geological knowledge of the basin

46

inversion, we can infer that the sediments under study have been overconsolidated and are likely to have been hardened. Although our model couldn’t exactly reproduce the SKM results, it is possible to constrain the ultimate expected subsidence of the basin between 192 and 1920 mm using SKM parameters and the no recovery scenario. Modelling can be used as a useful tool to predict the subsidence at various locations and assess the risk of getting coastal areas submerged under high storm waves. Reliability of the risk assessment will highly depend on the quality of the subsidence parameters used and availability of pressure drop and abstraction rates data. For the cases where subsidence measurements are available for a specific site, modelling can also be used to determine and better constrain the uncertain consolidation parameters of the sediments at that site. Modelling can be used to propose strategies to prevent areas at risks from getting submerged. The efficiency of various scenarios on limiting subsidence can be tested those might include:

1. A proposed abstraction cessation program; 2. Natural or induced recovery scenarios like injecting disposed

water in the oil production sites or storing CO2 to counteract the pressure head loss.

3. Strengthening of the highly compressible layers at a specific site. The successful injection of the strengthening material might be limited by the low permeability.

We recommend collecting samples from coastal areas suspected to be at risk of subsidence and conducting laboratory testing to determine the mechanical and hydraulic parameters. We also recommend studies to determine in situ stresses. In areas where subsidence measurements are available we recommend acquiring this data and utilizing it to determine the mechanical and hydraulic parameters. Modelling can be used then to calculate subsidence and assess risk of subsidence after comparison with the 100 yrs storm wave heights. Effect of geological structures and architecture on pressure changes at locations where no data is available can be accounted for once permeability data is available and is used in fluid flow modelling. Upon completion of the study, we will propose possible management strategies for fluid resources of the Latrobe Aquifer system in the Gippsland Basin.

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List of Symbols

σ Total stress; ; M/LT2 (e.g. MPa, Pa) σ' Effective stress; ; M/LT2 (e.g. MPa, Pa) σ'c0 Preconsolidation stress; M/LT2 (e.g. MPa, Pa) σ`fin Final effective stress; M/LT2 (e.g. MPa, Pa) σ`0 Initial effective stress; M/LT2 (e.g. MPa, Pa) ∆ Change in a property ν Poisson’s ratio; dimensionless γw Unit weight of water; M/LT2 (e.g. MPa, Pa) Eme Constrained Elastic Stiffness Modulus; M/LT2 (e.g.

MPa, Pa) Emv Constrained Plastic Stiffness Modulus; M/LT2 (e.g.

MPa, Pa) H Layer thickness; L (e.g. m, mm) ∆H Settlement; L (e.g. m, mm) H0 Initial layer thickness; L (e.g. m, mm) Em Constrained Stiffness Modulus; M/LT2 (e.g. MPa, Pa) E Elastic Stiffness Modulus; M/LT2 (e.g. MPa, Pa) Cv Consolidation index; (e.g. m2sec-1) t Time; T (e.g. sec, year) d Drainage path; L (e.g. m, mm) Tv Time factor; dimensionless U Average degree of consolidation; dimensionless P Pore pressure; M/LT2 (e.g. MPa, Pa) Sske Coefficient of elastic specific storage; L-1 ( e.g. m-1) Sskv Coefficient of plastic specific storage; L-1 ( e.g. m-1)

Note: subscript t stands for time, subscript 0 stands for initial value.

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GENERAL CONCLUSIONS This report forms the first of a series on projects that are aimed to address some of the recommendations of the Hatton report (Hatton et al. 2004). In particular, the Hatton report was based on a sampling of data from the on and off-shore Gippsland Basin, not the entire data set available, due to the high cost of data acquisition. While it was felt that the Hatton report had access to sufficient data to broadly characterise the basin-scale hydrogeological systems over time, there were data gaps identified both geographically and more generally for post 1990 times. In addition to these gaps in the formation pressure data, the Hatton report relied on published articles and reports that describe the main structural architecture of the basin for which there was some uncertainty. The Hatton report suggested that these structural features are important to understanding the pressure depletion response of the Latrobe Aquifer system to the various abstractions over time. In light of their importance, the Hatton report recommended that historical seismic data be examined to firstly confirm the location of major structural features and secondly attempt to evaluate the fault seal capacity. Finally, with regard to coastal subsidence risk, the Hatton report recommended a combination of monitoring and modelling to evaluate the risk of coastal inundation. The Victorian department of primary Industries has undertaken a 5 year multi site high resolution GPS monitoring program. A complimentary modelling program is required to predict subsidence risk across the area of concern. This model can subsequently be calibrated with observational data from Victorian DPI monitoring station data. In this phase of work, several of the above points have been addressed. In particular:

1) Additional pressure data collected for the offshore has both confirmed and sharpened the definition of the region and extent of aquifer pressure depletion. This now includes the area of Kingfish and West Kingfish. While no additional data for the time since1990 was available, an extrapolation of historic data in two key areas (Fortescue and Kingfish) was conducted to estimate the severity of present day depletion.

2) Examination of onshore seismic data has confirmed the location and extent of basin forming fault systems on the boundaries of the Seaspray Depression. Previously, these were suspected to be responsible for compartmentalizing parts of the Latrobe Aquifer system. This confirms that the geographic variation in the degree of pressure depletion observed in the Hatton report is likely related to faulted domains of the Latrobe aquifer. Within each domain, the cause of depletion can be different. Unfortunately, seismic data frequency and quality is insufficient to perform a complete shale gouge ratio determination on the fault zones for independent quantification of their seal capacity.

3) First stage simulation of subsidence, with parameters identical to the previous SKM model, produced similar but not identical

49

results (the CSIRO model predicted only half the subsidence of the SKM report). The most likely reason for the discrepancy is ambiguity on the preconsolidation stress. The sensitivity analysis has shown that uncertainty in the preconsolidation stress creates the most variation in predicted total subsidence. This uncertainty may be reduced if burial history curves for the basin can be determined.

Next Phase of Work Proposed In the next phase of work, it is proposed that the numerical subsidence model be expanded to incorporate the entire coastal region described in the Hatton report. Rather than using parameters to match those used by the SKM report, the actual 3-D pressure depletion profile characterized in this report will be input to the numerical simulation. Similarly, the actual stratigraphic and structural geology along the coastal zone will be built into the model. Realistic rock properties will be used, and the range of possible values (literature values) will be tested in order to allow for an optimistic and pessimistic prediction of subsidence. The model results will be linked in a Geographic Information System and overlain with a digital elevation model of the land surface, and the predicted future seal level changes and storm wave height data. The result will be a coastal subsidence risk map. In future, if funds are available to collect rock property data, the coastal subsidence risk map can guide the data collection to areas of highest risk. The numerical simulation shall include the possible mitigating impact of offshore CO2 sequestration that might allow a faster recovery of aquifer pressure. With the 3D numerical model complete, it can be calibrated against observations from the Victorian DPI high resolution GPS monitoring program. In the next phase of work, it is proposed that the numerical subsidence model be expanded to incorporate the entire coastal region described in the Hatton report. Rather than using parameters to match those used by the SKM report, the actual 3-D pressure depletion profile characterized in this report will be input to the numerical simulation. Similarly, the actual stratigraphic and structural geology along the coastal zone will be built into the model. Realistic rock properties will be used, and the range of possible values will be tested in order to allow for an optimistic and pessimistic prediction of surface subsidence. The model results will be linked in a Geographic Information System and overlain with a digital elevation model and predicted seal level and storm wave height data. The result will be a coastal subsidence risk map. In future, if funds are available to collect rock property data, the coastal subsidence risk map can guide the data collection to areas of highest risk. The numerical simulation shall include the possible mitigating impact of offshore CO2 sequestration that might allow a faster recovery of aquifer pressure.

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