Measurements and modeling of high-pressure adsorption of CH4 and CO2 on shales

Abstract

The Simplified Local Density (SLD) model has been widely used to study the adsorption of gases on activated carbon, coal and shale. However, previous studies mainly focused on the experimental data fitting and the adsorption prediction based on model regression parameters. Few studies have been conducted to investigate the fluid distribution in the pores, which is of great significance for the study of the adsorption mechanism. In this study, high pressure adsorption experiments coupled with SLD model were impressed on the study of adsorption characterizations of shales from Sichuan Basin, China. The results showed that the SLD model could fit the adsorption of gas on shale well, and the average absolute deviations were within 10%. The gas was adsorbed around the pore wall when the pore size was large. As the pore size decreased, the gas molecules in the pore center also began to be adsorbed and finally all the gas in the pores were in an adsorbed state when the pore size was decreased to a certain value. According to the adsorption potential energy curves of CH4 and CO2 gas, CO2 was preferentially adsorbed relative to CH4 when the pore size was larger than 0.42 nm, which was consistent with the experimental results; while when the pore size is less than 0.42 nm, CH4 was preferentially adsorbed. This may be due to that in smaller pores, the repulsion of the pores to CO2 was greater than CH4, making it more difficult for CO2 to enter the pores. The adsorbed phase density was a function of temperature, pressure and pore size. The temperature only changed the magnitude of the adsorbed phase density, while the pore size not only changed the magnitude, but also changed the tendency of the adsorbed phase density with pressure. A temperature dependency model was finally established. Adsorption at a wider range of temperatures can be predicted basing on the model.