Effect of pore structure on methane sorption potential of shales

Samples from the lower Paleozoic marine shale and Mesozoic continental shale in the west of middle Yangtze region were taken to investigate the effect of pore structure characteristics on the methane sorption of shale using field emission scanning electron microscope (FE-SEM), low pressure N2 isotherm analysis and high pressure methane sorption analysis. A mass of organic matter pores (mostly with pore diameter less than 50 nm) have been found in the high thermal evolution marine shale rich in organic matter. The positive correlation between TOC, N2 BET surface areas and sorption capacity in shale indicate that micro-porosity associated with organic matter is the key factor controlling methane sorption capacity of high thermal maturity shale rich in organic matter. The development of organic matter pores was limited in the organically lean marine shale and continental shale due to the lower TOC content and lower thermal maturity respectively, and their reservoir space is composed of inorganic matter pores in 30 nm to 4.5 μm diameter. Providing bigger specific surface area for methane sorption, the pores within or between clay particles is an important factor to affect methane sorption capacity. With the increase of thermal maturity, the major pore system of shale reservoir changes from inorganic matter pores to organic matter pores, favorable for the improvement of methane sorption capacity of shale.