Compaction, Permeability, and Fluid Flow in Brent-type Reservoirs Under Depletion and Pressure Blowdown by Øystein Pettersen, CIPR CIPR Technology Seminar 2010
Outline Experimental & Field Observations Theoretical Considerations Simulated compaction & permeability Influence on Fluid Flow
The Brent Group Schematic Overview Permeability 0 10D Sand strength TARBERT NESS T-3 T-2B2 T-2B1 T-2A T-1C T-1B T-1A N-3D N-3C N-3B N-3A N-2C N-2B2 High lateral continuity Silk sand areas Moderate continuity, low to moderate quality Small, irregular channels occur Extremely weak Very weak Moderate Weak Strong background, Weak channels N-2B1 N-2A N-1B N-1A Strong ETIVE E Good continuity Channel sands Moderate background, Very weak channels RANNOCH R-3 R-2 Low to moderate quality Upwards increasing permeability Increasing strength downwards R-1 BROOM
Examples Permeability vs. Load, Gullfaks 4500 4000 3500 1. load 1. unload 2. load 10000 1000 Loading Unloading 3000 2. unload K (md) 2500 2000 K (md) 100 1500 1000 10 500 0 0 10 20 30 40 50 60 p' (MPa) 1 0 10 20 30 40 50 60 70 80 p' (MPa) Permeability, md 6 000 5 500 5 000 4 500 4 000 3 500 3 000 300 295 290 285 280 275 Reservoir pressure (perm. press. gauge) Production tests 05/89-02/91 270 Lower Brent cores Upper Brent Well Tests Transient Analysis
Observations Data from a number of North Sea Brent Reservoirs show a permeability reduction of 20 95 % at a load increase of 100 bars Permeability reduction is (almost) irrecoverable Observations are in agreement with Grain Pack Model
Material Homogenization Permeability rate of change data from six different Brent Reservoirs dk/dp (md/mpa) 1000 100 10 WS US WS 1 WS 1 10 100 1000 10000 US WS 0,1 Initial permeability, K0 (md) Grain pack model: Rock under compression becomes increasingly harder to compress. In agreement with figure: High initial permeability perm.-reduction is larger for 1 bar load increase than when initial perm. is smaller. Hence, for two materials with initially significantly different permeability the permeability-ratio will approach unity as load is increased. Homogenization
Recapitulation of Previous Presentations 1. Compaction modelling in reservoir simulators is overly simplified, and not sufficiently accurate when compaction is an issue 2. Computation of accurate compaction requires coupled flow- and rock mechanics simulation 3. The most popular procedure is to do coupled simulations with iterative pore volume updates Very costly w.r.t computing time 4. We have developed a procedure whereby compaction can be computed without iterations, without loss of accuracy Introduces pseudo-materials and pseudo compaction vs. pressure relations
Test Cases ~Gullfaks petrophysics & material properties Grossly simplified geometry Very different properties in the various zones Channels in Ness 2 and Etive Channel widths: 15, 50, and 100 m Channel height: 4 12 m Large or moderate contrast in material strength between channels and background (cases CL & CM) Moderate to low vertical conductivity 16 years of moderate drawdown +22 years of maximum drawdown ( blowdown )
Examples Compaction & Channels PVM (Pore Volume Multiplier) 0.80 0.85 0.90 0.95 1.0 Compaction (by Pore Volume Multiplier) in Tarbert 2, in a West-East water drive. Traditional modelling: Compaction distribution resembles pressure distribution
15 m channels (16 m below) Examples Compaction & Channels (2) Correct modelling: (compaction computed from strain by stress simulator) Top: Load 50-100 bars Bottom: Load 150-200 bars
Examples Compaction & Channels (3) 15 m channels (16 m below) 50 m channels (16 m below) Correct modelling: (compaction computed from strain by stress simulator) Top: Load 50-100 bars Bottom: Load 150-200 bars Left: 15 m wide channels in Ness 2 Right: 50 m wide channels in Ness 2
Examples Compaction & Channels (4) 6 years Permeability multiplier in a (YZ) cross-section transverse to channels, near Upper Brent producers. Note vertical domain of influence from the channels 16 years 22 years Permeability multiplier 0 0.25 0.5 0.75 1
Variation of Permeability Multiplier Across Channel 1 Permeability multiplier 0.8 0.6 0.4 0.2 W = 15 m, MC W = 15 m, LC W = 50 m, LC Trad. model W = 100 m, LC 0-0.5 0 0.5 1 1.5 Normalized distance across channel
Homogenization (West-East X-section) Initial 18 years 20 years 24 years Permeability, md 3 7500 15000 22500 30000
Homogenization & Fluid Flow By homogenization we expect Permeability ratio channel background approaches unity Initially water will flow preferred through high-perm channels Without homogenization: Water cycling after breakthrough With homogenization: Preference to channels reduced (not that big difference between channel and b.g. permeability) Injection water spreads to background, better sweep
Water saturation in an Etive layer: No homogenization Constant permeability 6 years 28 years 32 years 38 years
Water saturation in an Etive layer: W. homogenization 6 years Dynamic permeability Constant permeability 28 years 32 years 38 years
Perm-multiplier in the Etive layer w. homogenization 6 years 28 years 18 years 32 years Permeability multiplier 0 0.25 0.5 0.75 1
Region Oil Efficiency 0.8 0.8 0.7 0.6 0.5 0.4 Trad K CL Const K CL Pseudo CM Pseudo CL 0.7 0.6 0.5 0.4 Pseudo Const K 0.3 0.3 0.2 0.2 0.1 0.1 0 0 10 20 30 40 Time, years 0 0 10 20 30 40 Time, years Ness 2 channels, 15 m channels Ness 2 channels, case CL, 50 m channels
Region Oil Efficiency, Etive, case CL, 100m Channels 0.9 0.6 0.8 0.7 0.6 0.5 0.4 Pseudo Const K 0.5 0.4 0.3 Pseudo Const K 0.3 0.2 0.2 0.1 0.1 0 0 10 20 30 40 Time, years 0 0 10 20 30 40 Time, years Channels Background
Well Oil Rates & Water Cut, Central Upper Brent Well Rate, Sm3/D 1000 800 600 400 200 Const K Pseudo Pseudo 1 0.8 0.6 0.4 0.2 Wcut, fraction 0 Const K 0 0 10 20 30 40 Time, years 50 100 m channels, Case CL
Comparison With & Without Blowdown 8000 Rate, Sm3/D 7000 6000 5000 4000 3000 2000 1000 0 q w Blowdown q w No blowdown q o No blowdown q o Blowdown 0 10 20 30 40 Time, years 50 100 m channels, Case CL
Summary 1. Permeability reduction in a sand / sandstone reservoir can be large even at moderate pressure drawdown 2. Compaction and permeability reduction can have large impact on fluid flow in a large class of reservoirs 3. Weak and strong materials behave differently when loaded, and by pressure reduction the initial permeability distribution can be altered; having strong influence on flow pattern 4. Reservoir deformation / compaction is more complex than traditional pressure-dependency-assumption. Rock mechanics simulations required
Summary, cont d 5. Coupled flow sim stress sim computation time has been significantly reduced by novel procedure 6. Material behaviour in a depletion or pressure blowdown process can contribute positively to recovery in many kinds of reservoirs 7. Factors influencing flow pattern change Perm. contrast strong weak materials Initial perm. in low-permeability materials Perm vs. load relationship Geometry (extent & distribution of strong / weak matr s Overall vertical conductivity