PORE CHARACTERIZATION AND METHANE SORPTION CAPACITY OF OVER-MATURE ORGANIC-RICH WUFENG AND LONGMAXI SHALES IN THE SOUTHEAST SICHUAN BASIN, CHINA

Currently, the upper Ordovician Wufeng (O₃w) and lower Silurian Longmaxi (S₁l) formations in southeast Sichuan Basin have been regarded as one of the most important target plays of shale gas in China. In this work, using a combination of low-pressure gas adsorption (N₂ and CO₂), mercury injection capillary pressure (MICP) and high-pressure CH₄ adsorption, we investigate the pore characteristics and methane sorption capacity of the over-mature shales, and discuss the main controlling factors for methane sorption capacity and distribution of methane gas in pore spaces.

Low pressure CO₂ gas adsorption shows that micropore volumes are characterized by three volumetric maxima (at about 3.5, 0.5 and 0.85 nm). The reversed S-shaped N₂ adsorption isotherms are type Ⅱ with hysteresis being noticeable in all samples. The shapes of hysteresis loop are similar to the H3 type, indicating the pores are slit- or plate-like. Mesopore size distributions are unimodal and mostly have diameters of 2-16 nm, which is consistent with MICP results. The methane sorption capacities of O₃w-S₁l shales are in a range of 1.63-3.66 m³/t at 30℃ and 10 MPa, and increase with the TOC content, surface area and micropore volume which suggest that organic matter might provide abundant adsorption sites for enhanced methane sorption. Samples with higher quartz content and lower clay content have larger sorption capacity. Our data confirmed that the effects of temperature and pressure on methane sorption capacity of shale formation are opposite to some extent, suggesting that, during the burial or uplift stage, the gas sorption capacity of hydrocarbon reservoirs can be expressed as a function of burial depth. Based on the adsorption energy theory, when the pore diameter is larger than 2 nm, most methane molecules will be adsorbed in pore spaces with a distance to pore walls less than 2 nm, while free gas is mainly stored in pore spaces further away. Distributions of adsorption space decrease with the increasing pore size, while free gas volume increase gradually, assuming the pores are either cylindrical or spherical. Particularly, when the pore size is larger than 30 nm, the content of adsorbed gas space volume is very low with a negligible contribution to total gas content.