OIL RECOVERY-2003, Moscow, May 19-23, 2003 1

AN INTEGRATED STUDY OF RESERVOIR FLUID FLOW ACROSS FAULTS BYLABORATORY EXPERIMENTS, 4D SEISMIC AND RESERVOIR SIMULATION

A. Stovas, M. Landrø, O. Torsæter, J. I. Jensen and J. Kleppe(Norwegian University of Science and Technology)

AbstractWater-saturated faults with high capillary entry pressure and very lowpermeability may hinder the flow of hydrocarbons in reservoirs due to formationof traps or fault-related compartments. The compartmentalization of thehydrocarbons can lead to lower than expected reservoir performance as has beenobserved in several oilfields in the North Sea. This paper presents an integratedstudy of such phenomena including laboratory experiments and modeling ofsmall-scale systems, and numerical simulations with generation of syntheticseismic of parts of the Gullfaks Field, and comparison with actual 4D seismic.The study shows experimentally that such blockage may be realistic at a smallscale. The blockage mechanism is reproduced at field scale by simulation of across-section in the Gullfaks Field with a fault located between a water injectionwell and an oil producer. Synthetic seismic is generated from simulation resultsand compared to actual 4D seismic for the field.

IntroductionThe purpose of this study is to investigate how various fault properties impact the fluid andpressure distribution in an oil field. A small-scale laboratory experiment is used to study thesealing properties of a water-filled fault. The field scale study focuses on a fault block in theI3 Area of the Cook Formation of the Gullfaks Field. Poor communication between theinjection well and the producer has been observed from production data. A fault is locatedbetween the wells, and the conductivity across the fault is uncertain. Several cases of differentfault transmissibility and capillary entry pressure have been investigated and compared withsynthetic and real time lapse seismic data.

Small scale studyThe small-scale study (Knudsen) includes laboratory experiments of two-phase flow(oil/water) and numerical simulations of flow behavior in models that mimic reservoirrock/fault block situations. The fault block was water saturated 100% and it was expected thatit would continue to act as a sealing fault/barrier to oil flow until the oil pressure in thereservoir zone exceeded the capillary entry pressure of the fault. A ceramic materialrepresented the fault rock while the reservoir rock had properties of Berea sandstone; bothrocks were characterized through special core analyses. The ceramic material can be viewedas a porous membrane across which fluid transport is obtained by a potential difference. Thedifferential pressure was achieved by injection of oil or water in the reservoir zone andgeneration of pressure drawdown by production through the fault zone. Relative permeabilityand capillary pressure data used in the simulation model Sendra were obtained by Coreyexponent representation in combination with mercury injection porosimetry. The resultsobtained from the simulations and experiments are given in Table 1. In Case I, the entire coreis water flooded. The pressure quickly reaches steady state, with a large DP across the fault,and a very small DP across the reservoir zone. Case II has no mobile water, and a fairly quickbreakthrough is seen. The krw in the reservoir zone is very low, and therefore no water fromthe reservoir zone will help in maintaining the water flow through the fault, generating a

OIL RECOVERY-2003, Moscow, May 19-23, 2003 2

faster breakthrough of oil through the fault compared to case III. In case III the water willflow more easily in the beginning, causing a slower breakthrough. When the water is injectedinto the case similar to case III (Case IV-1800), the water saturation profile changessignificantly. In case IV-1800 the breakthrough happens at a later stage than in case III. It canseem a bit strange since no oil is injected, only water. This leads to a lower inflow of oil intothe fault zone compared to III due to a higher krw in the areas next to the fault. The result isthe delay in breakthrough seen in Case IV-1800. In the remaining two cases (Case IV-1810and IV-2000), breakthrough can be seen at a quicker rate. Here the water phase is becomingmore dominant, and the water drives most of the oil out. From the experiments and numericalstudy we can conclude that the flow behavior of oil across the fault zone varied as a functionof both water saturation and the water injection rate in the reservoir zone. In addition, the oilbreakthrough was highly controlled by the permeability and thickness of the fault block.

Field simulationThe field scale study focuses on a fault block in the I3 Area of the Cook Formation of theGullfaks Field. A structure map with well locations and the location of the cross-sectionstudied is shown in Fig. 1. A schematic of the cross-section is shown in Fig. 2. Poorcommunication between injection well B-33 and production well C-1 is observed fromproduction data. A fault is located between the wells, and the transmissibility across the faultis uncertain. The observed pressure behavior in well C-1 for the time period 1990-2003 isshown in Fig. 3. In the simulation model used in this study, reservoir properties typical forthe Cook Formation are assigned to the reservoir unit on both sides of the fault. The faultitself is assigned very low permeability and 100% water saturation, and a drainage capillarypressure relationship with high entry entry pressure. The objective of the simulation is toreproduce observed pressures and to study effects of fault properties on field behavior. Theprinciple of fluid flow across a water-filled fault with high capillary entry pressure such as inthe system in Fig. 2 is that as oil production starts from well C-1, a pressure difference acrossthe fault develops. However, in the fault zone a drainage process will take place, and oilcannot displace water in the fault before the entry pressure is exceeded. Thus, there is delay inflow across the fault. Then, as water injection starts in well B-33 in the middle of 1997,pressure increases on the left side, accelerating fluid flow across the fault, and the pressure onthe right hand side increases.

The pressure behavior in the reservoir will depend on the capillary entry pressure level in thefault as well as on the fault permeability. An initial history match of the observed wellpressures in C-1 using a fault permeability of 0.025 mD and a capillary entry pressure of 10Bars is shown in Fig. 4. As may be seen, the match is quite good considering the uncertaintyin reservoir data. Several cases of different fault transmissibility and capillary entry pressurehave been investigated. Fig. 5 represents a fault permeability of 0.025 mD, and showspressures just left of the fault zone and just right of the fault zone for two different levels ofentry pressure, namely 10 Bars and 100 Bars. In the first case, a pressure difference across thefault of 10 Bars is reached almost immediately, and flow starts. After that, the pressuredifference increases to reach around 80 Bars at the start of water injection. The pressure at theright side increases by around 110 Bars in the time period between June 1 1999 and July 12001, the two time periods of 4D seismic. In the second case, a delay of around 2 years beforeflow across the fault starts is observed. Here, the pressure difference across the fault issubstantially higher, and reaches around 180 Bars when water injection starts. Again, apressure increase on the right side between the two 4D time periods of around 110 Bars isobserved. However, the pressure level is around 80 Bars lower than in the previous case.

OIL RECOVERY-2003, Moscow, May 19-23, 2003 3

Since fault permeability also will affect the pressure behavior, two additional cases weresimulated. On Fig. 6 results of three different fault permeabilities are presented, all for a entrypressure of 100 Bars. Significant differences in pressures for the three cases may be observed.In Fig. 7, observed pressures are plotted together with simulated pressures for entry pressureof 100 Bars, with changing fault permeability. Clearly, a match of the pressures may also beobtained at the higher entry pressure by choosing appropriate fault permeability. Thesimulations demonstrate that a high capillary entry pressure in a fault combined with a lowpermeability across the fault may explain observations in many oil fields of lack of flowbetween reservoir sections.

Time lapse seismic modelingThe simulation results yield pore pressures and fluid saturations for all grid cells as a functionof time. The estimated pore pressures in 1999 and 2001 are shown in Figure 8. We notice thesignificant increase in pressure at both sides of the fault between 1999 and 2001. To computeseismic parameters from the output reservoir parameters, two different rock physics modelshave been employed: A conventional Gassmann (1951) model is used for fluid substitution,and a modified Hertz-Mindlin model (Mindlin, 1949) is used to compute seismic parametersas a function of effective pressure. For simplicity, a one-dimensional reflectivity-basedmodeling is used. In this case, the reflection coefficient is defined by the change in acousticimpedance, and the time-shift is defined by the change in velocity. Both impedance andvelocity are saturation-pressure dependent (Stovas and Landrø, 2002a,b). The synthetic timelapse difference data is shown in Figure 9. We notice amplitude changes at the top reservoirinterface at both sides of the fault, however the amplitude change is more pronounced at theright side. Figure 10 shows amplitude maps for top reservoir in 1999 and 2001. Comparingthe real 4D seismic data with the synthetic difference data (the position of the synthetic linecorresponds to the red dashed line in Figure 1), we observe similar effects: There is anamplitude increase north of the fault from 1999 to 2001, and we can also see some evidenceof increased amplitude south of the fault from 1999 to 2001. However, there are also somediscrepancies: The observed amplitude increase north and south of the fault seems to be of thesame magnitude on the real data, while the synthetic seismic data suggests stronger amplitudeincrease south of the fault. Despite these quantitative differences, we might say that this studyindicates that the effects observed in laboratory experiments also can be seen in a producingoil field.

ConclusionsLaboratory experiments conclude that water filled faults with high capillary entry pressuremay effectively block fluid flow at low pressure differentials across the faults. Reservoirsimulation of a cross-section of the Gullfaks Field shows realistic scenarios of such blockagefor a variety of fault parameters on a field scale. Comparison of synthetic seismic generatedfrom simulated pressures and saturations with actual 4D seismic from the Gullfaks Fieldsupports the theory of fault blockage.

AcknowledgmentsWe acknowledge Statoil and their license partners Norsk Hydro and Petoro for permission touse data from the Gullfaks Field. Per Digranes, Lars Kristian Strønen, Ingvild Mæland andAnne Mette Irgens are acknowledged for support and several discussions.

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ReferencesKnudsen, E.:"Water saturated faults with high entry pressure: A laboratory and simulationstudy", M.Sc. Thesis (2002), Norwegian University of Science and Technology (NTNU),Trondheim, Norway.Gassmann, F., 1951, Elastic waves through a packing of spheres, Geophysics, 16, 673-685.Landrø, M., Digranes, P. and Strønen, L.K., 2001, Mapping reservoir pressure and saturationchanges using seismic methods – possibilities and limitations, First Break, 19, 671-677.Mindlin, R.D., 1949, Compliance of elastic bodies in contact, J.Appl.Mech., 16, 259-268.Stovas, A.M. and Landrø, M., 2002a, Fluid-pressure discrimination in anisotropic reservoirrocks – a sensitivity study, paper submitted to Geophysics.Stovas, A.M. and Landrø, M., 2002b, Optimal use of PP and PS time-lapse stacks for fluid-pressure discrimination, paper submitted to Geophysical Prospecting.Råsberg, E.: "Simulation of the Statfjord I1 block and application of the Gassmann equationfor conversion from reservoir parameters to seismic velocities", Semester project (2001),Norwegian University of Science and Technology (NTNU)Gassmann, F., 1951, Elastic waves through a packing of spheres, Geophysics, 16, 673-685.

Table 1. Data and results from the experimental and numerical study.

Sw ReservoirSw Faultzone

Initial systempress. (kPa)

Injectionpress. (kPa)

Productionpress. (kPa)

Breakthroughtime (min)

Case I 1 1 16001600 (waterinjection)

Atmosphericpressure

Test onlyno oil

Case II 0.1 1 18001800 (oilinjection)

Atmosphericpressure 5650

Case III 0.3 1 18001800 (oilinjection)

Atmosphericpressure 6100

Case IV -1800 0.3 1 1800

1800 (waterinjection)

Atmosphericpressure 6600

Case IV -1810 0.3 1 1800

1810 (waterinjection)

Atmosphericpressure 6100

Case IV -2000 0.3 1 1800

2000 (waterinjection)

Atmosphericpressure 2100

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B-33

C-1

Matched Pressure in Well C-1 for a Capillary Entry Pressure of 10 Bars and a Fault Permeability of 0.025 mD

0

50

100

150

200

250

300

350

400

1/1/89 1/1/91 1/1/93 1/1/95 1/1/97 1/1/99 1/1/01 1/1/03

DateP

ress

ure

(Bar

)

Fig. 1: Top Cook structure map. Locations ofinjection well B-33 producer C-1 are shown. Thecross-section used in simulation study is markedwith a dashed red line.

Fig. 4: History match of observed pressures inwell C-1 using an entry pressure of 10 bars and afault permeability of 0.025 mD

Effect of Capillary Entry Pressure in Fault on Pressure Response in Reservoir

0

100

200

300

400

500

600

700

800

1/1/89 1/1/91 1/1/93 1/1/95 1/1/97 1/1/99 1/1/01 1/1/03

Date

Pre

ssur

e (B

ar)

P th =100

Left side of fault

Right side of fault

P th =10

P th =10

P th =100

1/6/99 1/7/01

Fig. 2: Schematic cross-section used for re-servoirsimulation (red dashed line on Figure 1)

Figure 5: Pressure versus time for two differententry pressures left and right of the fault (dashedlines indicate times for 4D seismic).

Observed pressure in Well C-1

0

50

100

150

200

250

300

350

400

1/1/89 1/1/91 1/1/93 1/1/95 1/1/97 1/1/99 1/1/01 1/1/03

Date

Pre

ssur

e (B

ar)

Effect of Permeability in the Fault Zone on Pressure Response in Reservoir

0

100

200

300

400

500

600

700

800

1/1/89 1/1/91 1/1/93 1/1/95 1/1/97 1/1/99 1/1/01 1/1/03

Date

Pre

ssur

e (B

ar)

K f =0.025

Right side of fault

Left side of fault

K f =0.1K f =1.0

K f =0.025

K f =0.1

K f =1.0

1/6/99 1/7/01

Fig. 3: Observed pressures in well C-1 for the timeperiod 1990-2003

Fig. 6: Pressure vs. time for three fault per-meabilities at left and right side of the fault.

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Simulated and Observed Pressures in Well C-1 for Various Fault Permeabilities and a Capillary Entry Pressure of 10 Bars

0

100

200

300

400

500

600

1/1/89 1/1/91 1/1/93 1/1/95 1/1/97 1/1/99 1/1/01 1/1/03

Date

Pre

ssur

e (B

ar) K f =0.025

K f =0.1

K f =1.0

Fig. 7: History match of observed pressures in wellC-1 using a entry pressure of 100 bars and variousfault permeabilities

.

Figure 8a: Simulated pressures in 1999

0.00.2

0.40.6

0.81.0

1.21.4

0.030

0.035

0.040

0.045

0.050

1.851.90

1.952.00

2.052.10

2.152.20

June, 1999

Pre

ssu

re, G

Pa

Depth

, km

X, km

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Figure 8b: Simulated pressures in 2001

Figure 9: Simulated 4D seismic differences between 2001 and 1999. Notice strong amplitude increase(at top reservoir) at the right side of the fault, as well as significant amplitude change at base reservoir.

0.00.2

0.40.6

0.81.0

1.21.4

0.030

0.035

0.040

0.045

0.050

1.851.90

1.952.00

2.052.10

2.152.20

July, 2001P

ress

ure

, GP

a

Depth

, km

X, km

1.90

1.85

1.80

1.75

1.70

1.65

1.60

1.55

1.50

1.45

1.400.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

July, 2001 - June, 1999

Tim

e, s

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Figure 10: 4D amplitude map (top reservoir) showing significant amplitude changes close to theinjector (B-33) and south of the fault between the injector (B-33) and the producer C-1