A dilemma in calculating ethane absolute adsorption in shale gas reservoirs: A theoretical approach

• Absolute adsorption of ethane is calculated via hybrid GCMC/MD with various models. • The models used are Langmuir, Toth, and supercritical Dubinin–Radushkevich (SDR). • Despite a good fit none of the models can predict absolute adsorption precisely. • Adsorbed volume is preferred to adsorbed density to estimate absolute adsorption. Shale gas plays are composed of different constituent components (mineral and organics) and a mixture of fluids. To accurately estimate the gas in place (GIP) in such reservoirs, calculating the absolute adsorption quantity would be critical, particularly for ethane fluid as the second major component in the gas composition. In this study, for the first time, by employing hybrid grand canonical Monte Carlo/Molecular Dynamics (GCMC/MD) simulations, the behavior of ethane in graphene layers at various pore sizes (1, 2, and 4 nm), temperature (303.15, 363.15, and 363.15 K), and pressure ranges (0-50 MPa) is observed. Moreover, ethane and helium probes were employed to calculate the accessible volume followed by the absolute adsorption isotherm analysis based on Langmuir, Toth, and supercritical Dubinin–Radushkevich (SDR) adsorption models. Finally, the impacts of calculating the adsorbed density via the ethane density profile to convert the excess adsorption to the absolute adsorption isotherm was investigated. The results showed that despite a good fit of adsorption models with the excess adsorption isotherm, neither of the adsorption models can predict the absolute adsorption precisely. Thus, the better fit of the adsorption model with the excess adsorption isotherm alone cannot be a good criterion for calculating the absolute adsorption isotherm. Collectively, we proposed that adsorbed volume should be favored instead of adsorbed density to estimate the absolute adsorption.