Characterization of Whole-Aperture Pore Structure and Its Effect on Methane Adsorption Capacity for Transitional Shales

The structure of shale pores controls gas storage and gas transport. However, because the distribution of shale pore sizes is wide, it is difficult to use only one method to completely characterize the structural characteristics of shale pores. We employed various techniques to characterize the whole-aperture pore structure of 15 shale samples of different levels of maturity collected from a transitional shale formation of the Upper Paleozoic, Ordos Basin, China. Field-emission scanning electron microscopy (FESEM) image analyses indicated that interparticle pores between or within clay minerals and microfractures are commonly developed, while organic matter pores (OMPs) develop poorly, compared with those observed in marine shale. Highpressure mercury intrusion porosimetry (MIP) and low-pressure N-2/CO2 adsorption experiments showed that the pore size distribution (PSD) ranges from the nanometer scale to the micrometer scale and is multimodal. The dominant PSD ranges are 0.45-0.65 nm, 0.75-0.95 nm, 3-5 nm, 10-50 nm, 10-40 mu m, 2-50 nm (mesopores), and >50 nm (macropores). Mesopores and macropores provide most of the shale pore volume (PV), accounting for 42.9% and 52.7%, respectively. However, micropores only account for 4% of the PV, although micropores (<2 nm) and mesopores contribute most to the pore specific surface area (PSSA), accounting for 31.2% and 68.6%, respectively; the contribution of the macropores is almost negligible. Combined with CH4 isothermal adsorption experiments, the effects of the shale pore structure on the shale adsorption capacity were analyzed; a positive correlation was observed between methane adsorption capacity and the specific surface area of shale micropores, indicating that micropores are one of the main factors controlling the adsorption capacity of shale. The total organic carbon (TOC) content and clay minerals also contribute significantly to the methane adsorption capacity, while brittle minerals have negative effects on the shale adsorption capacity.