CARBON ENERGY PL

Managing Ground Deformation in UCG

Cliff Mallett

UNDERGROUND GASIFICATION of COAL

Why is Deformation an Issue?

• Commercial scale UCG removes similar coal volumes to a large longwall mine

• UCG Designs must provide consistent high volume gas production, and be viable with large scale extraction

• Cavity collapse results in induced permeability and subsidence– Changes gasification conditions with water inflow– Cause mixing in overlying aquifers – Surface impacts

Hoe Creek #3 Trial (USA, 1979)

Felix#2 (8m)

Clay (5m)

Felix#1 (3m)

Overburden (39m)

•Total of 11m of coal at 39-55m depth

Potential Environmental Impacts

Water Table

IncreasedPermeability

Surface subsidence

Water usage

Contamination

Typical UCG High Production Layout

Commercial scale may use 1 to 7 panels a year

200m

600m

Barrierpillar

UCGPanel

Surface pipework

What can be done about Deformation?

• Predict ground and water behaviour from site characterisation data

• Incorporate whole-of-life water flow into UCG operational designs

• Assess environmental impact models & monitor– Surface features– Groundwater systems

But we cannot stop deformation, and if impacts cannot be managed, we must abandon the site

for UCG

Understanding Deformation

• UCG is analogous to longwall mining– Comparable coal will be removed– Thickness of coal – ash left but more coal taken

• Learn from longwall mine behaviour– We know that deformation is severe immediately above a

cavity, but decreases in impact at higher levels• Apply verified longwall predictive models to UCG

eg COSFLOW– Stress and Strata movement– Fracture and Induced permeability– Fluid flow

Critical Implications for UCG

• Large scale shallow UCG extraction will– Open direct pathways from surface to gasification cavity– Disrupt groundwater requirements for gasification

• Minimum depth for large scale extraction 300m– Maintain 150m of undeformed buffer over disrupted strata– At depth. in seam drill holes better than vertical holes

• Above 300m only partial extraction is safe– This limits UCG gas production levels achievable

Managing Deformation

• Integrated site characterisation• Numerical modelling

– Mine Water – Mine subsidence

• Field monitoring • Subsidence control

Site Characterisation3D geotechnical model

Drilling, in-situ stress and

permeability measurements

3D sedimentary model Automatic drillholegeotechnical interpretation

Magnitude of Major Horizontal Stress - All Boreholes

0

100

200

300

400

500

600

0 5 10 15 20 25 30 35

Major Hor iz ont al St ress ( Mpa)

Nominal overburden SG = 2.5

Upper bound t rend with depthfor major horizontal st ress

In-situ stress

Automated Log Transformation

The computer program LogTrans is used to identify Coal, Mudstone, Siltstone and Sandstone from their unique petro-physical signatures using the density, natural gamma and sonic velocity geophysical logs.

The figure shows the geological interpretation of a control hole. The first column is the geological classification from the geologist. The second column is the geophysical conciliated geological classes for LogTrans’ training processing. The third column is the LogTrans interpretation from the geophysical logs presented in the other columns.

Density Gamma

From Borehole to Numerical Model

0.0

33.0

50.0

91.0

209.0

286.0294.0

224.0

364.0

AQ1

AQ2

AQ3

AQ4

AQ5

Hei

ght a

bove

the

min

ing

seam

(m)

???

???

???

???

Needs further measurement

150.0

209.0

0

-50

-100

-150

-200

-250

-300

-350

-400

-450

Mining Seam

In-situ water head = 123m

In-situ water head = 98m

In-situ water head = 58m

In-situ water head = 12m

Very limited information isavailable and needs further measurement

-9.0

HydrogeologyHydrogeology--Delineation of AquifersDelineation of AquifersWithin a mine lease (Within a mine lease (10Km X 10 KM10Km X 10 KM) )

26 PIEZOMETERS INSTALLED IN 10 DRILL HOLES

???

SPR31

LW40

9

LW41

1

LW41

3

LW41

5

LW41

7

SPR32

SPR33

SPR34

SPR35

SPR26

SPR27SPR29

Flow direction in AQ1Flow direction in AQ2

400 Panel

LW41

9

LW42

1

Local Flow DirectionLocal Flow Direction

Regional Flow

Direction

Pore pressure over panel

Change in permeability and

reservoir pressure

Mining induced strata fracture/deformation

Ground Water Flow Gas diffusion and flowChange in reservoir

pressure and relative permeability

Change ineffective stress

Vertical Displacement

Caved, fractured and deformed zones

INTERACTION

Numerical ModellingNumerical Modelling

COSFLOW

COSFLOW is a coupled dual porosity two phase flow model developed with a specific objective of addressing the mine issues, such as ground deformation, water flow and gas emission

Couples rock mechanics of layered strata with one or two phase compressible fluid flowCosserat Continuum => efficient simulation of the deformation behaviour of stratified rockEstimates rock fracture induced changes in hydraulic properties (e.g. permeability and porosity)Simulates water and gas flow through fractured rock

COSFLOW is a program developed By CSIRO and JCOAL & NEDO

COSFLOW Application

Modelled Vertical Stress & Microseismic data

COSFLOW Application

Vertical stress in a coal seam

COSFLOW Application

Hydrogeology

Predictions of mine water inflow made 2 yearsago are are within 10% of actual inflow

Pore pressure plots aroundlongwalls

Permeability prediction

Aquifer 1Coal seam

Aquifer 2

Successive longwalls

COSFLOW - SubsidenceLayout of West Cliff Colliery

-800

-400

0

400

800

1200

1600

2000

-820 180 1180 2180 3180 4180 5180 6180

5A5A 5A5B5A6 5A75A8 Measurement borehole

1

4

2

3

5(Excavation sequence)

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

-800 -300 200 700 1200 1700

Horizontal distance from LW 5A5, m

Subs

iden

ce, m

Standard model

Empirical prediction

Cosserata model

5A5 5A6 5A7 5A8 A sequence of longwall blocks

COSFLOW - SubsidenceL a y o u t o f W e st C l i ff C o l l ie ry

-8 0 0

-4 0 0

0

4 0 0

8 0 0

1 2 0 0

1 6 0 0

2 0 0 0

-8 2 0 1 8 0 1 1 8 0 2 1 8 0 3 1 8 0 4 1 8 0 5 1 8 0 6 1 8 0

5 A5 A 5 A5 B5 A6 5 A75 A8 M e a s u re m e n t b o re h o le

1

4

2

3

5 (E x c a va t io n s e q u e n c e )

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

-800 -300 200 700 1200 1700

Horizontal distance from LW 5A5, m

Subs

iden

ce, m

Standard model

Empirical prediction

Cosserata model

5A5 5A6 5A7 5A8

Prediction Vs Performance

COSFLOW - SubsidenceLayout of West Cliff Colliery

-800

-400

0

400

800

1200

1600

2000

-820 180 1180 2180 3180 4180 5180 6180

5A5A 5A5B5A6 5A75A8 Measurement borehole

1

4

2

3

5(Excavation sequence)

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

-800 -300 200 700 1200 1700

Horizontal distance from LW 5A5, m

Subs

iden

ce, m

Standard model

Empirical prediction

Cosserata model

5A5 5A6 5A7 5A8

Connectivity in GoafsTracer gas studies(1)

• Gas migration between adjacent longwall panels

• Longwall goaf –behaves as one system for gas

• Gas pressure and buoyancy effects –across all goafs

Managing Deformation Impacts

• Predict ahead– Reject area for UCG if unsuitable– Manage the issues

• Management plan for risks– Monitor insitu – know what is happening and have a

planned response for all identified risks – stop activity if it causes problems

• Active mitigation– Specific operational procedures– Innovative practices

Geotechnical monitoring

Microseismicmonitoring

Stress monitoring

Displacementmonitoring

Microseismic monitoring

3D location of microseismic deformation events & burn front positioning

Subsidence mitigation