Mechanical behavior and damage-induced permeability evolution of mudstone and gypsum caprocks

Abstract

Both mudstone and gypsum caprocks play a vital role in the accumulation and preservation of petroleum because of their low porosity and permeability. The transport property of caprocks changes due to the tectonic compression or uplift. The permeability evolution behavior may differ significantly between the two caprocks, however, they are not well-understood. Therefore, in this work, mudstone and gypsum samples were collected and a series of experiments were performed to investigate the permeability evolution at different stress conditions. The results show that the permeability of the original gypsum sample is one to two orders of magnitude lower than that of the original mudstone due to different texture of the rocks. The mudstone has bedding and clastic texture while the gypsum is crystalized. The permeability evolution in triaxial compression test after the sample failure increases up to two orders of magnitude for mudstone, while the permeability for gypsum does not increase and the final permeability is even lower than the initial permeability. Permeability evolution under unloading confining stress tests shows that it increases up to three orders of magnitude for mudstone and about one to two orders of magnitude for gypsum. The differences are mainly due to the different failure modes, brittle deformation for mudstone and ductile deformation for gypsum. Moreover, the generated fractures cross the mudstone samples while the generated shear bands do not cross the gypsum samples. A model was developed to describe the permeability behavior for both loading and unloading processes based on the change of volumetric strain.