understanding of reservoir compartmentalization and...

Understanding of Reservoir Compartmentalization and its Impact on Flow Behaviour inFractured Basement in Mumbai High South field.

Uma Goyal *, [email protected], S. N. Chitnis, B. B. Ray, ONGC, Mumbai, India,

Keywords

MHS, MDT, SBHP, DFN

Summary

Fractured Basement reservoir is unconventionalhydrocarbon reservoir where hydrocarbon storage andconductivity are provided by fracture system associatedwith faults. The average porosity of fractured reservoir issmall, about 2-3% while the permeability is distributed inrelatively wide range. Fracture system divides reservoirinto compartments and creates high heterogeneity ofpermeability and porosity distribution. Flow in fracturedbasement reservoir is unpredictable and requires in-depthstudy for reservoir characterization.

Compartment identification is an important factor in theexploration and exploitation of oil and gas reservoirs andmay have a significant impact on the estimation of in-placeand/or recoverable reserves, as well as the placement ofexploration, development and production wells.

Present study discusses the causes of compartmentalizationat structural level, from reservoir pressure data, fluidproperties and anomalies in Static model i.e. DiscreteFracture Network for different wells of prospective up dippart of Mumbai High South. Compartmentalization can becaused by lateral flow barriers, such as faults, reservoirpinch-out, or it may be caused by simple structural closurealong the top or base seal surface. Study considers analysisof pressure production data at basement level, fluidcharacteristics, seismic data, geochemical data and welllogs FMI data. This study has led to better understanding ofunconventional reservoir and its flow behaviour. Theanalysis has been useful in deciding the optimumplacement of new wells and relocation of poor producers.

Introduction

Mumbai High is a giant multilayered field situated in thewestern offshore of India. The field is producinghydrocarbons primarily from multilayered lower Miocenecarbonate reservoirs and the fractured Basement and BasalClastics as secondary reservoirs. Although commercialaccumulation of hydrocarbons from naturally fracturedBasement and Basal Clastics have been established quietearly. After over three decades of establishing hydrocarbonin Basal Clastic and Basement, focused efforts to exploitthe full potential of this unconventional reservoir havetaken pace in last five years through exploratory anddevelopment drilling. The study area is indicated inlocation map of Western Offshore Basin as shown in Fig. 1.

Geological Setting

Mumbai high field is a large doubly plunging anticline(paleohigh), bounded towards east by NNW-SSE trendingbasement controlled fault and towards west, it grades intomonoclinal deeper continental shelf. The entire shelf is splitinto longitudinal strips by a number of Basement controlledfaults, which resulted in horsts and grabens (Fig. 2).

The field is giant paleohigh of the Precambrian graniticrocks overlain by Deccan Traps at some part and clasticsand carbonates over the greater part of the area. Tertiarysedimentary sequences of Early Oligocene to recentoverlying the Basal Clastics were deposited in response tovarious transgressive and regressive events registeredduring geologic past. Eastern boundary fault is a majorzone of deformation comprising several faults which areoffset by minor ENE-WSW cross faults. The drilled welldata indicates the basement rocks of varied lithologyconsisting of granite gneiss, biotite schist, phyllites andbasalts. The major carbonate reservoirs of Mumbai highwere deposited during upper part of Early Miocene(Bombay formation) and Middle Miocene (BandraFormation). In between these two major carbonatereservoirs, a thin sheet like sand/silt body is deposited,which is designated as Mahim formation.

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Reservoir Compartmentalization and its Impact on Flow Behaviour in fractured Basement

Fig 2: Major Tectonic Trends in Mumbai High

The Mumbai High remained exposed till Oligocene times.The weathering due to longer exposures, alterations ofmineral constituents, faulting and fracturing by tectonicprocesses, might be some of the reasons for thedevelopment of secondary porosity and permeability, andthereby creating the reservoir.

Objectives

Basement reservoir of Mumbai High exhibits widevariation in production rates; wells drilled away from faultzones are devoid of hydrocarbons whereas some wellsdrilled over the structures or close to faults producedhydrocarbons in the range of 1500 to 2500 bopd.Structurally shallower wells found dry, produced oil withhigh water cut and structurally deeper wells found cleaneroil. Pressure transient analysis also indicated that very lessdissipation of pressure and depletion is around the wellbore. Phase behaviour studies also indicated the variation influid characteristics in the basement wells. The seismic dataand formation micro imaging data analysis indicated thatthe faults and fractures are associated with anomalies instatic model DFN studies. Hence, integrated approach ofanalyzing available G&G data with pressure and

production history of wells is essential for evolving aworking strategy to realize the potential of Basement.

Area of Study

The large size and complex reservoir characteristics of thefield have necessitated the acquisition of information,initially on a strategy to drill widely spaced exploratorywells to the Basement for determining the geological andreservoir heterogeneities, areal extent, fluid properties andthe initial oil and gas volumes. Later, vertical well of mostof the platforms were drilled to the Basement during thelate eighties for better understanding of the reservoir. Initialoil in place is estimated mainly in four areas A, B, C and Das shown in Fig. 3.

Fig 3: Established Hydrocarbon areas in Basement

The present work is based on analysis of geoscientific dataof Area B in MHS. The area B is considered because ofavailability of pressure and production data.

Exploitation1 status of Area - B in MHS

The area separated by a structural low which is bounded byprominent extensional cross faults trending nearly eastwest. The area is bounded to the east by the Mumbai Highmain extensional fault with over 600 m throw towards east.In the year 1989 with the drilling of exploratory well-ACreal breakthrough was achieved with higher oil productionaround 2500-4000 bopd in Basement (Fig.4). During 1990to 2000 two development wells produced oil in this area.Well-AB produced oil around 750 bopd during initial



testing and cumulative oil production was 0.058 MMt. Thewell was transferred to upper L-III due to low influx. Otherwell-AE was completed towards mid 90s and produced oilover 600 bopd with around 60% water cut. Well hascumulatively produced 0.011 MMt and it has also beentransferred to upper L-III due to High water cut.

Fig 4: Exploitation status of wells in Area-B

For further development1 of this area, one appraisal cumdevelopment location AD for Basement was drilled toBasement in 2009 as a directional well. The wellencountered near vertical fractures with very less apertureand some are filled with pyrite & quartz. The Basementsection produced only water and no influx was observedfrom Basal Clastics even after stimulation job.

In order to exploit the oil reserves of Basal clastics andBasement from the known Block of well-AC area, the well-AD as ADz was planned and drilled to a depth of over4000m (100m tvd in Basement). Well-ADz was drilledduring 2011-12, about 630m south of well-AC and put onproduction in April 2012. The well-ADz was completed inBasal clastics and Basement and produced around 1800bopd of clean oil on self flow with ½” choke and iscurrently producing oil around 800 bopd with ¾” chokewith 28 % water cut. One subsea well AC-SS has beendrilled and put on production in mid 0f 2014. Well AC-SSis producing @1400 bopd with no water cut. Completion ofwell-ADz & AC-SS added the oil production of over 3300bopd in this area and presently the area is contributingaround 2200 bopd with 15% water cut.

Methodology

The common goal is to characterize reservoir propertieswith sufficient resolution so that fluid flow pathways are“mapped”, production behavior predicted, and hydrocarbon

production maximized. Accurate characterization of theconnectivity and compartmentalization of permeablereservoir (reservoir flow-unit architecture) is required. Forthe last several decades it has been widely held that the useof sedimentary-process and sequence-stratigraphic analysiswould help resolve reservoirs architecture. However, theimpact of these techniques in unconventional reservoir hasbeen limited because they are based on non-dynamicdepositional facies models and an incompleteunderstanding of the range of possible reservoirarchitectures.Initial reservoir pressures obtained from Pressure Transientanalysis and Reservoir Formation testing indicatesanomalous pressure points such as reservoir depletion,inadequate build up periods and sub-seismic reservoircompartmentalization. Low permeability/porosity reservoirrocks, dramatic lateral changes in structural geometries,and large distances between wells generally producereservoir compartmentalization and can be ascertained asgiven below.

1. Structural Compartments effecting production potential2. Pressure Compartments3. Fluid properties anomalies4. Basement characterization by DFN showing

discontinuitiesResults

1. Structural top and production potential

In Area B structurally shallower2 wells like AD producedwater and well AE produced oil with high water cut andstructurally deeper wells like AC, ACSS & ADz producedgood amount of oil around 1500 to 2500 bopd as shown inFig. 5.

Fig.: 2 Established HC areas in Basement

Fig. 5: Hydrocarbon anomalies at Structure level



Wells in Area B exhibits wide variation in production ratesand at shallower level produced water or oil with highwater cut which indicates that compartments may existwithin the well and area which is effecting production frombasement. Data is shown in structure map and Table 1.

Table 1: Basement top and testing results

Production performance of different wells in Area B isindicating the wide variation in oil rate. Well ACSS andAdz started producing with 1500 to 1800 bopd. Wells ABand AE initially produced with around 650 to 750 bopd andlater ceased due to high water cut indicated in Fig. 6.

Fig 6: Production Performance of wells in Basement

2. Pressure compartments

Pressure-depth plots are powerful methods to visualize thewide array of pressure data and aid interpretation for safety,drilling, and exploration purpose and compartmentanalysis. Pressure compartmentalization of reservoirsinvolves restricted fluid movements over geological time.Initial reservoir pressure of wells in Area A indicates thatwells were drilled at different time from 1989 to 2015 butinitial reservoir pressure2 recorded was around 2950 psithough in between wells have produced oil from Basement.

Table 2: Initial Reservoir Pressure of wells

Multiwell P-T plots permit recognition of pressurecompartments in the subsurface. Wells where the samemagnitude of pressure is found at a common reservoir-levelare likely to be located in a shared pressure compartment(Fig. 7). Different amounts of pressure in the samereservoir in multiple wells indicate either the reservoir iscompartmentalized by barriers (largely static) or the fluidsare flowing from a higher overpressure region (dynamicsystem). A hydrodynamic system can be recognized wherepressures systematically change without the presence ofbarriers. Recorded pressure in producing wells is plottedwith time and cumulative oil production showing that eachwell is behaving like separate unit.

Fig 7: P-T Plots with cumulative oil

MDT data and recorded SBHP values of wells completedin Basement in MHS has been analysed and tabulated in theTable-2 which clearly shows that when a new well iscompleted , recorded pressure is close to initial reservoirpressure around 2950 psi. It is showing that with oilproduction pressure is depleting around the well bore (Fig.8) and it is a common observation that pressure is notdissipating within the field and area. Pressure data analysisclearly indicates compartmentalization of basement and

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ACSS Adz AE ABDate

Oil

Rate

bop

d

Variation in Production Performence in Area B

Well YearBasement

top tvd mslQl bpd Qo bpd

AB 1990 1881 746 746AC 1989 1899.5 2575 2575

ACSS 2014 1956.5 1532 1532AD 2009 1868 WaterAdz 2012 1885 1861 1861AE 1994 1864 1700 665



Basal Clastics reservoirs in the area and between wells.These compartments respond in different ways topressure/stress changes, and on flow behaviour whichbecomes a great challenge.

Fig 8: P-T Plot of wells in Mumbai High Field

3. Fluid properties Anomalies

Fluid circulation in the crust and in particular hydrocarbonmigration in reservoirs is highly dependent on faultgeometry and hydromechanical properties. Understandingthe evolution of these properties during fault growth andnetwork development is of major importance in fluid flowprediction. Phase behaviour2 of a system quantifies theinterplay between fluid properties (density and viscosity),Saturation pressure, Gas Oil ratio (GOR) value and fluidequilibration processes (flow driven by pressure or densitydifferences and diffusion) to reveal the existence ofcompartment in the reservoir.

Fig 9: Variation in PVT parameters of wells in differentareas

Phase behaviour studies of wells in Areas A, B, C and Dshow (Fig. 9) that saturation pressure of wells is rangingfrom 36 to 123 kg/cm2 and GOR values are also in widevariation from 50 to 81 v/v. Bottom hole temperature isalso varying from 1090 C to 1350 C. It is indicating thatfluid properties of wells are varying from area to area.

4. Basement characterization by DFN showingdiscontinuities

The understanding of the network of fractures is one of themajor challenges in unconventional basement reservoirs.The strategy of exploitation and the development of field isdetermined by the spatial distribution of these fractures.The zones with strong density of fractures, the knowledgeof their connectivity, orientation, inter fracture spacing andtheir elongations are the essential elements.

Fig 10: Discrete Fracture Network of wells in Area B

There are many situations where compartmentalizationunexpectedly impacts development; we still have some wayto go before we have work flows and practices that enableconsistent prediction of the impact of faults, fractures,stress and other reservoir heterogeneities ahead of thedrilling.The fracture properties, viz. fracture length, fracture width,and fracture density were up-scaled into the 3D geologicalmodel to create the Discrete Fracture Network (DFN)Model which captured the spatial distribution of fracturesand associated heterogeneities. DFN model helps to showmore clearly the intensity of the fracturing around the wells



(Fig. 10). Static model of this area is prepared with theacquisitions of well data by the imaging techniques andseismic data, which is reflecting the discontinuities.

Conclusions

The goal is to derive a general strategy to improve profitsthrough a better understanding of reservoir size,compartmentalization, and orientation as well as reservoirflow characteristics. Understanding the nature anddistribution of reservoir compartments, and using effectivereservoir management strategies, can significantly improverecovery efficiencies from Basement reservoir of MumbaiHigh field. The approach involves an integrated geologic,geophysical, and engineering approach devised to identifyheterogeneities in the subsurface that might lead toreduction of uncertainties. The approach involves thefollowing key steps:(1) Impact of structural complexity as seen from table-1(that shallower wells produced water); on reservoir fluidflow to guide further exploration and exploitation. Thishappens when wells failed to intersect connected,conductive fractures. It requires pre-drilling description ofthe effective fracture network.(2) Investigated trends of reservoir pressure in table-2

clearly indicate that wells near to each other do notcommunicate and fractures are working as barriers whichmay create reservoir compartment.(3) Basement reservoir in which fault & fracture networkscontrol on production, characterization of fracture networksis a vital task in optimizing the flow. DFN model is one ofthe best ways to characterize a basement reservoir. Itaccurately accounts for the heterogeneity and orientation offracture set. Thus it is vital to characterize reservoircompartmentalization as early as possible in field life,ideally during appraisal.As more wells are drilled, it has become possible to bettercorrelate and map the producing horizons with the properunderstanding of compartmentalization of the fracturedreservoir and study can also be extended to other producingareas of the field. This clearly allows better planning of thelocation of future wells, and the thickness of the basementsection which needs to be penetrated, for optimizing fieldeconomics.

References

1. Goyal Uma et al (2015) Exploitation of Hydrocarbonsfrom Unconventional Fractured Basement/ Basal Clasticreservoirs underlying the offshore giant Mumbai HighField.2. Well Completion and Formation Evaluaton, PVT reports,and pressure build up study of various wells from MumbaiHigh field-unpublished reports of ONGC.

Acknowledgments

The authors would like to thank the ONGC authority forgranting the permission to publish the data and the findingsof the study through this paper. The Authors express theirgratitude to Shri. A.K.Diwedi, Director (E) for providing anopportunity, encouragement and support. The authorsexpress their sincere thanks to all fellow colleagues ofCOD Basement Exploration and MH Asset for theirsupport and assistance.


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