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Fractures and fluid flow in Fractures and fluid flow in petroleum reservoirs petroleum reservoirs

Quentin Fisher

Centre for Integrated Petroleum Engineering and GeoscienceSchool of Earth and Environment

University of LeedsE-mail: quentin@rdr.leeds.ac.uk

cipeg.leeds.ac.uk

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OutlineOutline• Drilling wells in petroleum reservoirs can cost

>$200 million

• The success of those wells is often dependent on predicting how faults and fractures are affecting fluid flow in the subsurfac

• Key issues: -– Faults: conduits vs barriers

– Fractures: predicting their distribution

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Faults as barriers to flowFaults as barriers to flow

(from van der Molen et al., 2003EAGE conference on seals, Montpellier)

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Fault rocks as flow barriersFault rocks as flow barriers

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Faults as conduits for fluid flow

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Faults as conduits for fluid flow

Mud losses

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Conduits Conduits vsvs barriersbarriers• Active vs inactive

– Often argued that active (or critically stressed) faults are conduits whereas inactive (or below critical stress) are barriers

– Many examples where seismicity is not associated with significant fluid flow

– Most published examples where critically stressed faults increase flow are from hard rocks

• Rheology vs stress conditions– Barriers when formed when deformation is ductile– Conduits often form when deformation in brittle– C.f. critical state soil mechanics

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FaultsFaults in clean Brent sandsin clean Brent sands

Time of deformation

Dep

th

Brent

Faults

Rotliegendes

Cataclasties

Dev

onia

nbr

ecci

as &

fract

ures

Rotliegendes

cemented fa

ults

& fractu

res

Start of quartz cementation(~90 C)o

Columbianfaults

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RotliegendesRotliegendes: cataclastic faults: cataclastic faults

Time of deformation

Dep

th

Brent

Faults

Rotliegendes

Cataclasties

Dev

onia

nbr

ecci

as &

fract

ures

Rotliegendes

cemented fa

ults

& fractu

res

Start of quartz cementation(~90 C)o

Columbianfaults

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Faults in Colombian sandstoneFaults in Colombian sandstone

Time of deformation

Dep

thB

rent

Faults

Rotliegendes

Cataclasties

Dev

onia

nbr

ecci

as &

fract

ures

Rotliegendes

cemented fa

ults

& fractu

res

Start of quartz cementation(~90 C)o

Columbianfaults

Faulting of very low porosity rocks (<15% porosity) often from conduits not barriers to flow

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Fault Fault brecciabreccia –– Devonian Reservoirs Devonian Reservoirs North SeaNorth Sea

Time of deformation

Dep

th

Brent

Faults

Rotliegendes

Cataclasties

Dev

onia

nbr

ecci

as &

fract

ures

Rotliegendes

cemented fa

ults

& fractu

res

Start of quartz cementation(~90 C)o

Columbianfaults

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Faults as conduits Faults as conduits vsvs barriers in clean barriers in clean sandstones: sandstones: ‘‘rulerule--ofof--thumbthumb’’

Consistent with observation, critical state theory of soilmechanics, experimental rock mechanics and numerical modelling.

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Key uncertaintiesKey uncertainties• Impact of fault rocks on fluid flow over geological time-

scale

• Controls on the brittle-ductile transition in shales

– Lack of experimental data

• Calibration of fault seal prediction methodologies

– Most ways of calibrating fault seal predictions are intrinsically non-unique

• Complexity

– Fault zones tend to be very complex but we model them in a very simple way – is this OK?

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Fracture predictionFracture prediction• Mode I fractures often act as conduits for fluid flow in petroleum

reservoirs• Often flow is highly localised

– In fractured reservoirs, a huge proportion of oil production often comes from very thin intervals in a small proportion of wells

• Predicting the presence/distribution of fractures is intrinsically difficult– Fracture models often don’t work – but could we do better?– Are seismic methods best way of predicting fracture distribution?

• How do we predict the long term (>50 years) changes in fracture permeability as a function of change in reservoir pressure (tight gas sands)

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Top seal capacityTop seal capacityFilled to spill

Under-filled

Empty

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Capillary sealsCapillary seals

2R

Pc = 2γcosθ/R

Top seals have small pore-throat sizes and therefore can act as capillary seals

Where:

Pth = threshold pressure (psi)σ = interfacial tension (Dynes/cm)θ= contact angleR = pore throat radius (microns)

RPc

θσ cos2=

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Leakage along Leakage along hydrofractureshydrofractures

• Pore pressure needs to overcome minimum horizontal stress while leakage occurs

From Nordgård Bolås and Hermunrud, 2003

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Problems with existing methodologiesProblems with existing methodologies• The petroleum industry is not very good at predicting

top seal failure– OK at post-mortems

• Do we actually know how top seals leak?• Can we predict top seal leakage during reinjection of

CO2? • Could seismic help?• What modelling should be used?

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Stress path during reStress path during re--inflationinflation

σij – effective stressSij – total stressα – Biot coefficientp – pore pressure

ijijij pS δασ −=

pSΔΔ

= 33γ

• Estimates of stress path have been made from repeated leak-off tests during depletion

• Some evidence that stress paths can be lower during inflation than deflation (i.e. fracture pressure is lower)

From Santarelli et al., (SPE, 47350)

Stress path parameter

Effective stress

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Modelling fracture etcModelling fracture etc

Synthetic seismicCoupled geomechanical – production simulation models

MP

I interface

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Valhall Field Valhall Field -- BackgroundBackground

(from Barkved, 2003) (from Kristiansen, 1998)

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• Seismic anisotropy is the directional dependence in seismic velocities- Indicator of order in a medium- Indicator of style of flow, stress regime or fracturing

Seismic anisotropy & shear wave splitting

Shear-wave splitting

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Fracture size estimation using frequencyFracture size estimation using frequency--dependent sheardependent shear--wave splitting.wave splitting.

After Maultzsch et al. (2003); EAP work

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Yibal

ValhallValhall: • Results for overburden• Low amount of anisotropy• No obvious freq-dependent anisotropy

Yibal: • Results for carbonate reservoir.• Clear freq-dependent anisotropy

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ConclusionsConclusions• We seem to be making significant progress predicting

the fluid flow properties of faults in petroleum reservoirs– Calibration is difficult due to non-uniqueness of evidence– More work is needed to understand how faults affect

fluid flow in sediments such as shales• Predicting the distribution of open fractures in the

subsurface is more challenging– Fracture models often don’t work– Maybe seismic methods are more promising– Could we improve our modelling?