Simplified In-Situ Stress Properties in Fractured Reservoir Models. Tim Wynn AGR-TRACS

Transcription

1 Simplified In-Situ Stress Properties in Fractured Reservoir Models Tim Wynn AGR-TRACS

2 Before the What and the How is Why Potential decrease in fault seal capacity Potential increase in natural fracture permeability Early assessment of the influence of in-situ stresses Relatively fast model construction and analysis Iterations with standard reservoir simulators Ownership by asset team members Use results to guide more specialised modelling Simplified? Stress properties are static, no coupled modelling

3 Fundamental Principles - Normal and Shear Stresses σ = S - P p σ σ n = Normal Stress High values act to close fractures σ n τ τ = Shear stress High values act to move fractures σ Shear stress is not affected by pore pressure

4 The Fundamental Principles - Critical Shear Slope = µ ~ 0.6 Shear Stress τ µ = Coefficient of sliding friction CFF = τ (µ * σ n ) CFF = Coulomb Failure Function If CFF > 0 slip can occur Normal Effective Stress σ n Critical levels of shear stress on fractures leads to small slip events. Can enhance permeability through brecciation and breaking of minor mineral bonds

5 Building The In-Situ Stress Model Input Properties for each cell or fracture Principal stress magnitudes and orientations Pore pressure Fracture dip and dip azimuth In-situ stress modelling Output Properties for each cell or fracture Normal Stress σ n Shear stress τ CFF

6 Cellular Model Example Platform carbonate oil reservoir with small gas cap. Some compositional grading. Main structures formed in thrust environment Fractures required for permeability and some storage Current in-situ stress regime unclear could be normal or strike-slip Pore pressures are over-pressured. Higher gradient at top of reservoir. Stress regime changed from normal to strike-slip

7 Cellular Model Example Normal Stress Input Parameters Virgin Pore Pressure S H Magnitude S H Orientation Fracture Dip Azimuth

8 Cellular Model Example Normal Stress Output Parameters σ n Magnitude τ Magnitude Coulomb Failure Function

9 Cellular Model Example CFF Correlations 1 Outlined circles = strike-slip Un-outlined circles = normal

10 Cellular Model Example CFF Correlations 2 Outlined circles = strike-slip Un-outlined circles = normal

11 Cellular Model Example Conclusions CFF nearly always negative with modelled values Some indications that higher CFF values are associated with inflow However, many locations where higher CFF values are not associated with PLT inflow or mud loss intervals. Possible correlation with larger scale faults Further analysis on the correlations of parameters (fractures, faults, dynamic data, mineralisation, CFF) Further simplified modelling is warranted, possibly leading to fully coupled geomechanical modelling

12 DFN Model Example Platform carbonate gas reservoir Main structures formed in strike-slip environment Fractures may influence permeability and storage Current in-situ stress regime probably strike-slip Reservoir under appraisal, no production data Preliminary run of in-situ stress parameters

13 DFN Sector Model Workflow Create geocellular sector model in area of interest Populate with conventional reservoir parameters Populate with insitu stress parameters eg. Sv, SHmax etc Create DFN within geocellular grid volume Sample relevant grid parameters onto DFN fractures Calculate normal and shear stresses, CFF etc on DFN Modify fracture permeabilities using stress parameters e.g. CFF Upscale DFN: Porosity Permeability Sigma Repeat upscaling on different geocellular grids if required

14 DFN Sector Model Geocellular Grid Lithology Limestone layers Shale and evaporite interlayers Dolomite with best matrix properties Seismic scale fault 1 km

15 DFN Sector Model Geocellular Grid Properties ESE Fracture Intensity Lithology Controlled Fracture Dip Azimuth ESE dominated

16 DFN Sector Model Fracture Network Layer bound joints & Veins with facies dependent distributions Fault damage zone fractures Orientation distributions same for each set in all layers ~350,000 fractures 1 km Seismic scale fault

17 DFN Sector Model Property Sampling Pore pressure In geocellular grid Pore pressure Sampled into DFN

18 DFN Sector Model DFN Properties CFF on DFN fractures CFF modified permeability 350 SHmax

19 DFN Sector Model DFN Stress Modified Permeability CFF on DFN 015 SHmax CFF modified permeability 350 SHmax

20 DFN Sector Model DFN Stress Modified Permeability K k K j K i Upscaled CFF modified Kj permeability CFF modified permeability 350 SHmax

21 DFN Model Example Conclusions 25 change in SHmax direction made more difference than other stress parameters (SHmax, Pp etc). CFF values very low Too deep in structure? Stress magnitudes poorly constrained. Not critically stressed Mainly useful for modelling scenarios. Little dynamic data to constrain options. Some further simplified modelling warranted.

22 Conclusions 1. Simplified approach allows better integration of insitu stress data into the standard reservoir modelling workflow 2. Can help define uncertainties for constraining additional data gathering or further modelling 3. Valuable screening tool prior to more advanced geomechanical modelling 4. In-situ stress parameters available as preliminary input to wellbore stability or fracture design