Abstract
The proper representation of faults in coupled hydro-mechanical reservoir models is challenged, among others, by the difference between the small-scale heterogeneity of fault zones observed in nature and the large size of the calculation cells in numerical simulations. In the present study we use a generic finite element (FE) model with a volumetric fault zone description to examine what effect the corresponding upscaled material parameters have on pore pressures, stresses, and deformation within and surrounding the fault zone. Such a sensitivity study is important as the usually poor data base regarding specific hydro-mechanical fault properties as well as the upscaling process introduces uncertainties, whose impact on the modelling results is otherwise difficult to assess. Altogether, 87 scenarios with different elastic and plastic parameter combinations were studied. Numerical modelling results indicate that Young’s modulus and cohesion assigned to the fault zone have the strongest influence on the stress and strain perturbations, both in absolute numbers as well as regarding the spatial extent. Angle of internal friction has only a minor and Poisson’s ratio of the fault zone a negligible impact. Finally, some general recommendations concerning the choice of mechanical fault zone properties for reservoir-scale hydro-mechanical models are given.
Highlights
Hydro-mechanical simulations have developed into a standard tool for various subsurface applications ranging from hydrocarbon and geothermal reservoirs to underground storage sites for CO2 [1,2,3,4]
The base model (BM) comprises a fault zone with material properties typical for siliciclastic fault zones separating a sandstone reservoir, whereby the fault acts as conduit for fluid flow between different reservoir compartments
The corresponding mechanical material properties for a fault zone in such a setting are inferred from literature [24,40,43] and set to a Young’s modulus of 10 GPa, a Poisson’s ratio of 0.25, a cohesion of 10 MPa and an angle of internal friction of 25◦
Summary
Hydro-mechanical simulations have developed into a standard tool for various subsurface applications ranging from hydrocarbon and geothermal reservoirs to underground storage sites for CO2 [1,2,3,4]. Challenges exist regarding the proper implementation of faults into such numerical models. Pore pressure changes due to injection or production can induce slippage and fault reactivation, respectively [5,6,7,8]. This may cause induced seismicity, land subsidence and well collapse [9,10,11,12]. Fault reactivation may breach the reservoir seal causing up-fault leakage and allowing fluid migration due to enhanced permeability inside the fault zone [13,14,15]. Proper incorporation of faults into hydro-mechanical models is of crucial relevance for various reasons
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