Investigation of the Effect of Rock Mechanical Properties and In-Situ Stresses on Seismic Velocity Through a Coupled Geomechanical Reservoir Model

Abstract

SummaryThe dynamic poroelastic model identifies how dynamic elastic properties (P-wave and S-wave velocities and density) change during reservoir production. This forms the basis of time-lapse seismic feasibility studies. To provide accurate and meaningful predictions, the model requires appropriate input from reservoir engineering (pore-pressure and saturation variations) and geomechanics (stress and strain variations). In stress-sensitive reservoirs, a geomechanical simulation, coupled with the conventional reservoir simulation, updates vertical deformation and provides the mean-effective-stress field in and around the reservoir. The key objective of this work is to identify how combined reservoir and geomechanical effects will influence predictions of the dynamic poroelastic model.A method integrating reservoir engineering, geomechanics, and rock physics is applied to a 3D synthetic case. The first task of this study is to model deformation and stresses induced by exploitation within the reservoir and surrounding formations. For this, the pore pressure extracted from the reservoir simulation is introduced into the geomechanical model. Then, a sensitivity analysis with different mechanical parameters (Young's modulus, Poisson's ratio) and with different conditions (initial effective-stress ratio, plasticity behavior) is performed on a 3D model that includes the reservoir and its surroundings. The results of this geomechanical modeling are analyzed by considering the compression effect, the stress arching effect, and the mean effective-stress value in and above the reservoir. Then, the mean effective stress resulting from each geomechanical simulation enables us to update seismic velocities and the time shifts associated with seismic horizons, using Hertz-Mindlin's model. Last, this approach, considering the geomechanical aspect, is improved by the contribution of the fluid substitution on the dynamic poroelastic model, taking the saturation effects into account. The geomechanical approach is compared with some classical nongeomechanical approaches commonly used. This work demonstrates that the geomechanical approach influences the results in and around the reservoir. The time-shift values of the seismic horizons produced using this approach would be detectable on 4D-seismic data, and in this case study, the effect is much stronger than the saturation effect alone.