Evaluation of absolute adsorption isotherm in shale gas reservoirs

Estimation of absolute adsorption in shale gas reservoirs is one of the key parameters. Experimental studies can only measure the excess adsorption isothermal directly. Also, in most experimental studies, excess adsorption up to a pressure of 15 MPa is measured. As a result, Langmuir and supercritical Dubinin–Radushkevich (SDR) adsorption models are used to convert the excess to absolute adsorption at measured pressures and above. Using hybrid grand canonical Monte Carlo/molecular dynamics (GCMC/MD) simulations, we simulate methane fluid in kerogen with 4 nm pore size at three temperatures of 303.15, 333.15, and 363.15 K up to a pressure of 50 MPa. Then the accuracy of adsorption models to estimate the absolute adsorption is investigated. The molecular simulation results show that the adsorbed density is a function of pressure and temperature and is always less than the liquid methane density. The Langmuir and SDR adsorption models show low accuracy of both models in estimating the absolute adsorption at all temperatures. Finally, using the adsorbed volume obtained from molecular simulation to estimate the absolute adsorption at all temperatures and pressures has an error of less than 10%, and the use of this method is recommended.