ABSTRACT:

The paper presents a discussion on the strain-induced anisotropy in permeability for deformable granular media. A coupled deformation-flow-heat transfer simulator, which has incorporated the strain-induced permeability model, was developed using finite element method (FEM). It was used to conduct a coupling analysis of two-dimensional non-isothermal single-phase fluid flow in elastic porous media. As well, two fluid injection tests were carried out to investigate the effects of the permeability anisotropy. It is shown that the strain-induced permeability anisotropy does have significant impacts on the pressure response. It is also found that the proposed permeability model can accurately reflect the directional increase in permeability during fluid injection.

1. INTRODUCTION

The dependence of permeability on direction or permeability anisotropy in porous media has been confirmed by many field studies. The existing reservoir simulators usually deal with permeability anisotropy in such a way that the permeability in the principal directions may vary in magnitude at different locations; however, the orientations of the principal permeability remain the same throughout the reservoir. In reality, both the magnitude and the orientation of the principal permeability may vary from region to region in the reservoir due to the variation of effective stress in the reservoir formation. The permeability anisotropy mentioned in the following context is referred to the latter case.

Deformations in a reservoir are induced by the changes of pore pressure and temperature due to fluid injection and production in thermal recovery processes, thereby affecting permeability. However, in the geomechanics and petroleum literature, the permeability change of reservoir formation subjected to deformation changes is usually assumed as a function of porosity or volumetric strain, which is a scalar variable. Thus, the changes in permeability are equal in all directions even though the changes in strains are different in each direction.

Wong [1] analyzed the grain fabric of intact and sheared oil sand specimens using the thin section imaging method. He observed that even in intact natural oil sand specimens, the hydraulic radius and tortuosity factors vary in vertical and horizontal directions resulting in an intrinsic anisotropy in permeability. Based on theoretical and laboratory works, he developed a new permeability model for deformable porous media. This model assumes the tensor permeability is governed by induced principal strains. It can quantify the changes in permeability when the material experiences shear deformation and the changes in permeability can be anisotropic.

Coupled geomechanics-reservoir simulation is necessary in order to account for deformations due to pore pressure and temperature changes resulting from production and fluid injection. Conventional reservoir simulators usually use finite difference method (FDM) and assume permeability either isotropic or diagonal tensor. It is impractical to develop coupled geomechanics-reservoir simulators based on FDM numerical schemes due to its complexity. A coupled deformation-fluid flow-heat transfer simulator using finite element method (FEM) was developed. The full tensor permeability and the strain-induced permeability model were implemented into the simulator. It was then used to conduct a coupling analysis of two-dimensional non-isothermal single-phase fluid flow in elastic porous media. As well, two fluid injection tests were carried out to investigate the effects of the permeability anisotropy.

This content is only available via PDF.
You can access this article if you purchase or spend a download.