Volume Effects on Methane-Shale Adsorption under Reservoir Conditions

Abstract

Abstract Due to the low porosity of a shale reservoir, the amount of stored free gas is limited. The pore volume of shale is majorly composed of micropores and mesopores, which are perfect storage spaces for sorbed gas. As a result, a shale gas reservoir contains a significant amount of sorbed gas through physical adsorption. The adsorption behavior of shale is usually analogue to coal, although the average diameters of nanopores and compositions of shale are different from those of coal. Many adsorption models of shale and coal have been proposed in previous studies. No matter which model is chosen to describe an isothermal adsorption, the absolute adsorption is derived from the measured excess adsorption data. The basic calculation theory of absolute adsorption assumes that volume changes are only caused by adsorption. However, volume changes are also caused by adsorbent swelling or shrinkage, gas absorption into organic matter, and Helium adsorption. In this work, a modified adsorption model is introduced to account for the unexpected volume effects and accurately calculate absolute adsorption. The proposed model is based on the Dubinin-Astakhov (DA) model (pore-filling theory), since it is more suitable to describe the gas adsorption behavior in nanopores than the Langmuir Model that mainly describes the monolayer adsorption on homogeneous surfaces. Methane is a supercritical fluid under a high pressure and temperate reservoir condition. As a result, the pressure terms in the DA model are replaced by the density to overcome the problem of the determination of saturation pressure. The volume change term is additionally added to the adsorption equation to account for the effect of volume changes. Six groups of shale adsorption data are applied to validate the proposed model and analyze the volume effects. The proposed model shows better fitting results than the traditional DA model. Especially, the proposed model captures abnormal adsorption behavior under the extreme high pressure and high temperature conditions. The previous reported adsorbent swelling or shrinkage and gas absorption into organic matter are illustrated and explained by the calculated volume change terms. The adsorption capacity and other fitted parameters estimated by the proposed model are more reasonable and accurate than those predicted by the DA model. This study highlights the effect of volume changes on absolute adsorption calculation, and provides an extensive insight on shale gas adsorption behaviors under reservoir conditions.