Study of methane adsorption in calcite mineral pores in shale gas reservoirs by molecular simulations

An accurate estimation of methane absolute adsorption in nanopores of shale gas is crucial for a good estimation of gas in place (GIP). However, experimental studies would only provide us with the excess adsorption isotherm directly. In this regard, knowing the adsorbed density is necessary to calculate the absolute adsorption. For this purpose, most researchers calculate the absolute adsorption isotherm via the Langmuir adsorption model with a constant value for the adsorbed density. In the present study, using hybrid grand canonical Monte Carlo/molecular dynamics simulations, we explained how to improve the calculation of the adsorbed density in calcite. For this purpose, methane inside calcite mineral with a pore size of 4 nm at temperatures of 30 and 90 °C and pressures up to 50 MPa is simulated, and the effects of temperature and pressure on the amount of adsorption and adsorbed density are investigated. This study showed that adsorbed density increases and decreases with increasing pressure and temperature, respectively. The results verify that the Langmuir adsorption model with constant adsorbed density will underestimate the absolute adsorption capacity, which is exacerbated with pressure. Finally, the adsorbed density obtained from molecular simulations to convert excess adsorption to absolute values can provide acceptable results that can be improved the GIP assessments.