Investigation of Stress-Dependent Permeability Related Problems for Shale and Gulf of Mexico Deepwater Reservoirs with In-House Coupled Flow-Geomechanics Simulator

Abstract

Reservoir compaction and stress changes could have considerable impacts on reservoir management and production performance under certain circumstances. To consider geomechanics effects and provide more realistic dynamic reservoir simulations, we have developed an in-house mathematical simulator coupling fluid flow and geomechanics behaviors on FORTRAN. The coupled simulator was validated by comparing with the analytical solutions of the Terzaghi’s and Mandel consolidation problems. In this study, the developed coupled simulator is applied into four various reservoir applications, where unique physical mechanisms and additional geomechanics effects are added into the simulator. Firstly, various stress-dependent permeability correlations and matrix shrinkage phenomenon are taken into account for the coupled model to investigate their impacts on permeability change during reservoir depletion and production performance for organicrich shale reservoirs. Based on different rock properties and compaction behaviors, various stress-permeability correlations are separately applied into different sub-pore media (organic matter, inorganic matter, and natural fractures). Secondly, the coupled model usually encounters a large matrix system and high computational expenses for large-scale simulation problems, where the time stepping is a crucial factor for numerical stability and computational efficiency. We introduce an adaptive time stepping method with the modified local error technique to reduce iteration time and improve the computational efficiency for the coupled flow and geomechanics model. Thirdly, the permeability reduction derived from Pressure Transient Analysis (PTA) appears more severe than the permeability decline measured from core samples for Gulf of Mexico (GOM) Deepwater turbidite reservoirs. Based on the provided laboratory measurements and recorded-field data, we present a comparison study between laboratory-measured and field-derived permeability loss under compaction effects. Irreversible compressibility and permeability hysteresis are proposed to explain the difference with the support of numerous simulation results. Fourthly, correct measurement of stress-dependent permeability is critical for production prediction and economic evaluation of shale reservoirs. However, stress creep and effective stress coefficient still present difficulties in correctly measuring and interpreting stress-dependent permeability for cores-based measurements. An improved stress-dependent permeability model is derived to consider the effect of time-dependent compaction behavior on permeability measurements by incorporating the stress creep mechanism. Additionally, how to correctly interpret stress-dependent permeability results with appropriate effective stress coefficient is introduced in detail.