This paper focuses on investigating the impact of Steam Assisted Gravity Drainage (SAGD) operations on caprock stresses. Steam injection, in SAGD operations, results in significant pore pressure, temperature, stress and volumetric changes in the reservoir. One outcome is the deformation and stress variations of the overburden strata that could lead to the containment breach of the caprock through shear or tensile failure.
We developed a model based on the nucleus-of-strain method to assess the reservoir volume changes caused by the combination of reservoir shear dilation, thermal expansion, and increased pore pressure during the SAGD operations. We then developed a forward geomechanical model and used the reservoir volume changes as input data. The nucleus-of-strain model was verified by comparing the measured surface heave data with the same from the forward model.
The geomechanical model was used along with the Mohr-Coulomb failure criterion to assess the stresses and failure potential of the overburden strata. The model was applied to a SAGD reservoir for which surface heave data were available. The outcome indicates the potential for caprock integrity problems due to tensile fracturing. A parametric study was also performed to investigate the sensitivity of the predictions to various significant parameters including the Young's modulus, Poisson's ratio, in situ stress gradient, and reservoir depth, among others. The outcome of the parametric studies will be discussed in this paper.