SORBED METHANE DENSITIES IN ORGANIC-RICH GAS SHALES

Sorption is a complex and important process in surface chemistry. Understanding its behavior will help to better characterize the performance of engineered materials (e.g., catalyst) and protect personal health (e.g., respiratory protection). Black shale, a heterogenous geomaterial, has played an increasingly important role in the global energy market in the past decade. A significant fraction of gas in such shales is adsorbed on the surfaces of the exceedingly small pores. To estimate the total initial gas-in-place, experimentally measured ‘excess’ sorption is utilized to determine the absolute, or ‘real’, amount of sorbed gas. In this process, the density of the sorption layers is required. The pressure dependence, composition, and volumes of sorption layers in shales, however, remain poorly constraint. In this work, we propose a novel and practical experimental procedure to estimate the dynamic effective densities of sorbed methane.

To do so, a series of high-pressure CH₄ sorption isotherms were conducted with several Lower Permian Shanxi shale core samples from the Ordos Basin, NW China and one Poshidonia shale sample from Germany using a gravimetric method at three temperatures of 65, 75, and 95 °C. Further, organic geochemistry, X-ray diffraction mineral composition, and analyses of specific surface areas were carried out to shed light on the distribution of sorbed methane molecules on different shale minerals. We compared the estimated effective densities of sorbed methane at different pressures and temperatures under assumptions of monolayer and binary layer with cubic and hexagonal close packing. The results show that at the same temperature and low pressure, the effective density of sorbed methane increases with pressure within a monolayer, while at high pressure, sorbed methane will begin to accumulate beyond monolayer. The pressure at which the binary sorption occurs is determined by the sorption potential of shale mineral surfaces, especially that of the organic matter, indicating that at high pressure, the Langmuir model may not be appropriate. The ratio of sorbed methane to bulk methane decreases nonlinearly with pressure, which is also associated with the distribution of mineral surfaces. The hexagonal close packing provides a more reasonable explanation for the experimental measurements.