An experimental investigation of diffusivity and porosity anisotropy of a Chinese gas shale

Gas shale is fine grained and layered reservoir with strong anisotropy. The permeability of the fractures and diffusivity of the matrix are important flow parameters for shale gas production. Both parameters maintain close ties with shale pore structure and are affected by its anisotropy. Hence, study of anisotropy of shale pore structure and diffusivity is an important aspect in evaluating the production behaviour of shale reservoir. Previous laboratory studies on anisotropy were usually performed on core samples taken from different orientations on the same rock block; however, the sample difference may impact on the results of anisotropy, especially when the rock is heterogeneous. In this paper, a new experimental method is presented to investigate the anisotropy of shale porosity and diffusivity. Instead of using different core samples, cube samples are prepared and used to study the diffusion and porosity anisotropy. A shale sample from early Paleozoic strata in Hunan province, China was studied. Multiple cubes were cut from the shale block. The results show that diffusivity is much higher at the parallel to the bedding direction than that at perpendicular direction. Furthermore, porosity measurement also showed different results measured at different directions, suggesting that there are pores only accessible from one direction and the results suggest more such pores in the parallel to bedding direction. Moreover, different cube samples show different anisotropy even they were cut from the same shale block, suggesting that the shale block is heterogeneous and the cube approach is better to study anisotropy on heterogeneous rocks. Finally, scanning electron microscope (SEM) was used to study the pore structure and the relationship between the SEM observation and the diffusivity and porosity results was discussed.