Permeability variations associated with shearing and isotropic unloading during the SAGD process

This paper discusses the variations of reservoir permeability within unconsolidated sands due to isotropic and deviatoric (shear) processes during the SAGD process. Isotropic unloading results from the steam injection pressure, which is generally higher than the initial reservoir pressure. The high steam injection pressure results in the increase of pore pressure and reduces the confining effective stress within the drained zone and part of the partially drained zone. The shearing process is induced primarily by changes in total stress but can also occur with increased pore pressures. High steam temperatures associated with the SAGD process result in significant volumetric expansion of the reservoir material within the steam chamber. Thus, total stress is increased and the shearing process may occur beyond the steam chamber surface. Both the isotropic unloading and shearing processes can induce reservoir permeability variation because of the change in pore space, pore shape, and pore throat. For isotropic unloading, the configuration of the grains or their relative position is, for the most part, unchanged and the grains simply move apart without relative rearrangement. Whereas the shearing process induces substantial relative motion of the grains and significant changes in pore geometry. Based on lab test results, it is summarized that the isotropic unloading process produces much smaller changes in volume, absolute permeability, and water effective permeability compared to the shearing process. Consequently, incorporating stress-induced permeability change within coupled reservoir geomechanical simulations requires different relationships or models for these two conditions. In addition, some empirical permeability relationships, such as the Kozeny-Carman model, Tortike's equation, and Chardabellas's terms, are also discussed.