fractures and fluid flow in petroleum reservoirs · cipeg fractures and fluid flow in petroleum...
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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: [email protected]
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?