Absolute Adsorption of CH4 on Shale with the Simplified Local-Density Theory

Abstract

Summary Using a thermogravimetric (TGA) method, the excess methane (CH4) adsorption of four organic shale samples is measured at temperatures of 303.15 to 383.15 K and pressures of 0 to 15.0 MPa. Simplified-local-density (SLD) theory is used to calculate the density distribution of CH4 in nanopores, which is then used to obtain the adsorbed CH4 density on four shale samples. Such density is applied to obtain the absolute CH4 adsorption by correcting the measured excess CH4 adsorption. SLD theory shows that the adsorbed CH4 density is strongly affected by temperature and pressure, as well as the pore size, which is in line with the previous findings from molecular simulations. SLD theory captures the density of the adsorbed phase of CH4 in the presence of CH4/pore-wall interactions. However, the SLD theory is more efficient than molecular simulation methods in determining the adsorbed CH4 density considering that only two parameters in the SLD model are adjusted to match the excess adsorption of CH4 on shale. It is observed that the corresponding absolute adsorption of CH4 is higher than the excess adsorption; this suggests that it is not reasonable to use the measured excess adsorption to estimate the storage of CH4 on shale. This study applies the SLD theory to investigate the adsorption behavior of CH4 in organic pores at different pressure/temperature conditions, and, more importantly, it yields a more-efficient approach (i.e., SLD theory) in determining the absolute adsorption than the sophisticated molecular simulations tools.