Research on stress-sensitivity of fractured porous media

Abstract

Stress sensitivity parameters of a fracture are often tested under compression on both sides of the fractured medium, which is considered as an elastic solid. However, the elastic solid assumption will fail when two fracture surfaces, if applied to tensile stresses, are out of contact. In such cases, effective stress in the fracture is zero and the change of fracture aperture is not determined by the fracture itself, but by the deformation of the matrix and the overall deformation of the fractured region. This paper proposed a zero-stress model that is applicable after the failure of elastic fracture model as described above. The model analyzes the deformation mechanism of fractures and derives a new equation that can be used to describe the aperture and permeability behavior of a fractured medium under the condition of zero stress. The model is derived for different geometric shapes of fractured region under hydrostatic pressures. The model equations can be used to calculate aperture and permeability changes caused by the production of oil and gas from a reservoir as well as the injection of water or gases for EOR. Sensitivity analysis of the model found that both shape factor of the fractured region and elastic modulus of the surrounding rocks have a significant impact on the outcome of the aperture and permeability forecast as a function of changing pore pressure. The permeability model can also be used in reservoir engineering analysis and reservoir numerical simulation. The new model is joined with the traditional elastic model to better describe the deformation process of fractures.