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1、Faults as Fluid Flow Barriers and Their Role in Trapping Hydrocarbons Suzanne CooganSuzanne CooganRichard Nice Richard Nice Ayeni GboyegaAyeni GboyegaKate Carter-WalfordKate Carter-WalfordIntroductionFault seal mechanismsFault seal mechanismsInfluence on hydrocarbon fieldsInfluence on hydrocarbon fi
2、eldsModelling and Flow PropertiesModelling and Flow PropertiesCase StudiesCase StudiesHow Can Faults Become Barriers to Fluid Flow?JuxtapositionJuxtaposition of lithologies with different permeabilities across the of lithologies with different permeabilities across the faultfault Smearing Smearing o
3、f impermeable/semi-impermeable fault rocks in the fault of impermeable/semi-impermeable fault rocks in the fault zoneszones CataclasticCataclastic grain-size reduction results from abrasion during grain-size reduction results from abrasion during deformation;smaller grains fill pore space and reduce
4、 porositydeformation;smaller grains fill pore space and reduce porosity CementationCementation of damage zone by precipitation of solutes of damage zone by precipitation of solutes A complex fault zone will exhibit varying A complex fault zone will exhibit varying transmissibilitytransmissibility va
5、lues in values in three dimensions and these will change with displacement through three dimensions and these will change with displacement through timetimeJuxtapositionJuxtapositionJuxtaposition of lithologies with different permeabilities across the fault of lithologies with different permeabiliti
6、es across the faultJuxtapositionCoarse grained material(light colour)Fine grained material(dark colour)Analogous to a reservoir being juxtaposed against a sealing lithology Cataclasis CataclasticCataclastic grain-size reduction results from abrasion during grain-size reduction results from abrasion
7、during deformation;smaller grains fill pore space and reduce porositydeformation;smaller grains fill pore space and reduce porosity Effectiveness depends on:Effectiveness depends on:The hardness of the intact rockThe hardness of the intact rockThe magnitude of displacementThe magnitude of displaceme
8、ntInitial normal stress on the fault surface prior to movementInitial normal stress on the fault surface prior to movementGrain Size and Sorting ReductionFeldsparAlterationClayMineralFormationCataclasisShalesmearingSealing Mechanisms:Mica OrientationProcessesShearingFluidflowCementationMinerals carr
9、ied in solution in Minerals carried in solution in water under high pressurewater under high pressureAs fault opens,pressure release As fault opens,pressure release occurs,water flows through occurs,water flows through fault and minerals precipitate fault and minerals precipitate out of solutionout
10、of solutionCrystallisation of cements in pore Crystallisation of cements in pore spaces reduces permeabilityspaces reduces permeabilityCementReduced grain sizeClay SmearingLayersofshalecontainedwithinsequencearedrawnintofaultasmovementalongplaneprogressesImpermeablelayerformedalongfaultdependingonSh
11、aleGougeRatio(SGR)ratioofsandtoshaleShale Gouge Ratio This measure is useful for predicting sealing ability of fault.18 30%indicates high probability of a sealThe SGR is the percentage of shale within a part of the sequence which has moved past a point on the fault surface Shale Gouge ExampleShale G
12、ouge RatioAs SGR increases,sealing ability improves.The clay has a small pore throat size and therefore high capillary entry pressureWith smaller gouge ratios,brittle fracture and therefore cataclasis dominates.Sealing is less effective than clay smearExamples of Clay SmearingTheseexamplesshow3fault
13、sinoutcropthatrangefromsand-pronetoshale-pronegougeandanintermediatesand/shaleratiogouge.ThesefaultsdemonstrateaspectrumofgougecompositionandofsealbehaviourClay Smearing on Microscopic ScaleEffectiveness of Fault Sealing Mechanisms Sealing Capacity of FaultsH Hd d=2 2g gh h(r(rt t-1-1-r-rp p-1-1)/g)
14、/g(r rww-r rh h)r rt t=pore throat radius in the seal=pore throat radius in the sealr rp p=pore throat radius in the reservoir=pore throat radius in the reservoirg gh h=hydrocarbon-formation water interface tension=hydrocarbon-formation water interface tension (Oil:5-35 dynes/cm Gas:30-70 dynes/cm).
15、(Oil:5-35 dynes/cm Gas:30-70 dynes/cm).r rww=density of the formation water(1 1.2 gm/cm=density of the formation water(1 1.2 gm/cm3 3)r rh h=density of the hydrocarbon phase =density of the hydrocarbon phase(Oil:0.5 1.0 gm/cm(Oil:0.5 1.0 gm/cm3 3 Gas:0.2-0.4 g/cmGas:0.2-0.4 g/cm3 3)g g=acceleration
16、due to gravity=acceleration due to gravityModelling Empirical methods for risking the sealing potential of faults have been Empirical methods for risking the sealing potential of faults have been devised in combination with outcrop and laboratory studiesdevised in combination with outcrop and laboratory studies Estimates the sealing potential of a fault offsetting a particular sequence Estimates the sealing potential of a fault offsetting a particular sequence and therefore the entry pressureand
