Permeability Variations Associated With Shearing and Isotropic Unloading During the SAGD Process

Abstract

Abstract 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. Introduction The SAGD process has become a commercially viable technology to develop the vast oil sands resources in Canada. Reservoir simulation is used as valuable design tool for development projects and to predict SAGD production performance. However, in some instances, conventional reservoir simulation may not be suitable for predicting performance because it does not take into account the coupled mechanisms between fluid flow and reservoir deformation. In these cases, a coupled reservoir geomechanical simulation may be required. A requirement for these coupled simulations is suitable relationships between reservoir permeability and geomechanical behaviour. Oldak owski conducted a series of tests based on both reconstituted oil sands specimens and drilled oil sands cores to characterize the relationships of permeability and geomechanical processes(3).Permeability variations of sandstone and other high strength materials experiencing the isotropic unloading and shearing processes have also been studied(2). Chalaturnyk summarized test results on oil sands and provided the relationship between oil sands compressibility and confining effective stress(3). It is known that oil sands samples can be easily disturbed during the sampling process. In order to minimize the disturbance, Touhidi-Baghini(4) took the test specimens from an exposed outcrop of bitumen-free Mc- Murray Formation sandstone, northeast of Fort McMurray. Based on these specimens, experimental studies were conducted on the permeability variation during the shearing process. This paper systematically discusses the oil sands permeability variations due to the isotropic unloading and shearing process based on Oldakowski's and Touhidi-Baghini's test results. In addition, some empirical permeability relationship