In this study, we conducted numerical simulations of steam injection into a horizontal well. A sandy shale layer with thickness of 1 meter is overlain 2 meters away from the injection well. The numerical simulation aims at studying the variations of the stress state and the permeability in the sandy shale layer during steam injection, and to provide useful effective stress paths for laboratory experiments to follow when testing material properties of the sandy shale.
Sandy shale formations of several meters in thickness frequently exist in oil sands deposit. Their material properties, sedimentary characteristics and distribution in a SAGD reservoir are important factors that determine SAGD performance and strategies of positioning horizontal well pairs. It has been confirmed that sandy shale formation is usually very sandy and discontinuous; therefore, they are expected to be permeable to variable degrees during SAGD operations. This is completely different from the shale interbed which has extremely low permeability and can be considered as impermeable.
Analysis of stress state change and anisotropic permeability variation in the sandy shale requires conducting coupled simulation of geomechanics and thermal reservoir flow. Numerical modeling of the coupled processes is historically carried out in the areas of geomechanics modeling and the reservoir simulation. Gutierrez and Lewis(1) extend Biot's theory to multiphase fluid flow in deformable porous media. Based on their formulation, they conclude that the coupling between the geomechanics and the multiphase flow occurs simultaneously. Thus, fully coupled system equations of deformations, multiphase flow and heat transfer should be solved simultaneously. Development of such kinds of fully coupled geomechanics-multiphase flow-heat transfer simulators needs tremendous effort, since the existing FEM geomechanics codes and the FDM reservoir simulators cannot be used.
Settari and Mourits(2) present an approach to couple the stress-strain behavior to multiphase flow, heat transfer using porosity as a coupling parameter. The geomechanics module and the thermal reservoir simulator are used in a staggered manner. Pore pressure and temperature changes are calculated from the thermal reservoir simulator and transferred to the geomechanics module. The stress and the displacement changes are then calculated in the geomechanics simulation. An iterative algorithm is used to ensure that the porosity calculated from the geomechanics module is the same as that from the thermal reservoir simulator. The staggered technique employed to solve the coupled system equations allows for the use of the existing geomechanics codes in conjunction with a standard reservoir simulator. Currently, most of the commercial coupled geomechanics-multiphase flow-heat transfer simulators are developed in this way. The disadvantage of these kinds of coupled simulators is that the thermal reservoir module, usually developed using finite difference method FDM) cannot accommodate the full permeability tensor, since they adopt the standard discretization scheme such as 5-spot for 2-D problems and 7-spot for 3-D problems.
Much effort has been made in developing a coupled geomechanics-reservoir simulator using finite element methods (FEM) by Du and Wong (3, 4, 5, 6, 7, 8). The developed simulator has the capability of handling full permeability tensor and strain-induced permeability model.
