Compaction, Permeability, and Fluid Flow in Brent-type Reservoirs Under Depletion and Pressure Blowdown

Transcription

1 Compaction, Permeability, and Fluid Flow in Brent-type Reservoirs Under Depletion and Pressure Blowdown by Øystein Pettersen, CIPR CIPR Technology Seminar 2010

2 Outline Experimental & Field Observations Theoretical Considerations Simulated compaction & permeability Influence on Fluid Flow

3 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

4 Examples Permeability vs. Load, Gullfaks load 1. unload 2. load Loading Unloading unload K (md) K (md) p' (MPa) p' (MPa) Permeability, md Reservoir pressure (perm. press. gauge) Production tests 05/89-02/ Lower Brent cores Upper Brent Well Tests Transient Analysis

5 Observations Data from a number of North Sea Brent Reservoirs show a permeability reduction of % at a load increase of 100 bars Permeability reduction is (almost) irrecoverable Observations are in agreement with Grain Pack Model

6 Material Homogenization Permeability rate of change data from six different Brent Reservoirs dk/dp (md/mpa) WS US WS 1 WS 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

7 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

8 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 )

9 Examples Compaction & Channels PVM (Pore Volume Multiplier) Compaction (by Pore Volume Multiplier) in Tarbert 2, in a West-East water drive. Traditional modelling: Compaction distribution resembles pressure distribution

10 15 m channels (16 m below) Examples Compaction & Channels (2) Correct modelling: (compaction computed from strain by stress simulator) Top: Load bars Bottom: Load bars

11 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 bars Bottom: Load bars Left: 15 m wide channels in Ness 2 Right: 50 m wide channels in Ness 2

12 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

13 Variation of Permeability Multiplier Across Channel 1 Permeability multiplier W = 15 m, MC W = 15 m, LC W = 50 m, LC Trad. model W = 100 m, LC Normalized distance across channel

14 Homogenization (West-East X-section) Initial 18 years 20 years 24 years Permeability, md

15 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

16 Water saturation in an Etive layer: No homogenization Constant permeability 6 years 28 years 32 years 38 years

17 Water saturation in an Etive layer: W. homogenization 6 years Dynamic permeability Constant permeability 28 years 32 years 38 years

18 Perm-multiplier in the Etive layer w. homogenization 6 years 28 years 18 years 32 years Permeability multiplier

19 Region Oil Efficiency Trad K CL Const K CL Pseudo CM Pseudo CL Pseudo Const K Time, years Time, years Ness 2 channels, 15 m channels Ness 2 channels, case CL, 50 m channels

20 Region Oil Efficiency, Etive, case CL, 100m Channels Pseudo Const K Pseudo Const K Time, years Time, years Channels Background

21 Well Oil Rates & Water Cut, Central Upper Brent Well Rate, Sm3/D Const K Pseudo Pseudo Wcut, fraction 0 Const K Time, years m channels, Case CL

22 Comparison With & Without Blowdown 8000 Rate, Sm3/D q w Blowdown q w No blowdown q o No blowdown q o Blowdown Time, years m channels, Case CL

23 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

24 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