Characterization of pore microstructure and methane adsorption of organic-rich black shales in northwestern Hunan, South China

The gas adsorption of shale is one of the most important factors influencing shale gas accumulation and production. Different from other shale gas fields, the shale in Northwestern Hunan province with strong structural deformation has especially high portion of adsorbed gas. The adsorption abilities of shale reservoir play a significant role in the success of exploration and production of shale gas in Northwestern Hunan. Therefore, based on many experiments including field emission scanning electron microscopy, N2/CO2 adsorption, excess CH4 adsorption with constant temperature and others, this paper found that: 1) The shale pores were identified with mineral matrix pores, organic matter pores and fracture pores, and the pores can be further classified according to their shape features and forming mechanism; 2) After analyses of the distribution of the pore size, Mesoporous is the largest contributor and widely distributed, and micropores provide more contribution on surface area but limited contribution to pore volume. Meanwhile, depth has little effect on the properties of shale reservoirs; 3) Based on the fractal geometry theory, surface roughness is positively correlated to microstructure irregularity, which means the effects between the gas adsorption by Van der Waals force and by multi-layer or capillary condensation in single pore system are consistent during the N2 adsorption process. 4) The abilities of gas storage are apparently sensitive to the surface roughness of shale pores and non-sensitive to microstructure irregularity. TOC has strong positive correlation with the surface roughness, specific surface area, total pore volume and Langmuir volume, which means TOC can improve the adsorption of shale reservoir in many ways. Pyrite can offer much contribution to the ability of gas adsorption.