Massive steam injection during a steam assisted gravity drainage (SAGD) operation results in significant changes in pore pressure, temperature, stress and volumetric strain in the reservoir sand. These changes trigger the deformation and stress redistribution of the overburden strata, which could lead to containment breach of the caprock through shear or tensile failure. Iso-tropic models have commonly been used to assess caprock integrity. These models are believed to be deficient in capturing the anisotropic response of shale and mudstone, which has been well documented in applications other than SAGD caprock studies.
We developed an iteratively coupled hydro-thermo-mechanical model to evaluate caprock integrity in SAGD reservoirs by coupling a geomechanical model (FLAC) and a thermal reservoir simulator (CMG STARS). We investigated the impact of SAGD operations on stress distribution in the caprock using an anisotropic model that we implemented in FLAC. We repeated the analysis by using the same tool and model, but with the assumption of isotropic shale, and compared the results. Incorporating anisotropy in the model showed an impact on the state of caprock stress and failure pressure of the caprock. It was concluded that shale anisotropy cannot be neglected in caprock integrity investigations.