Estimation of adsorbed-phase density of methane in realistic overmature kerogen models using molecular simulations for accurate gas in place calculations

Methane adsorption in the organic matrix (kerogen) of gas shales is an important factor in determining the storage of natural gas and volumetric calculations of gas in place. The pore system in shales is very heterogeneous, ranging from micropores, mesopores, and macropores, and the pore size affecting the adsorption behavior. For example, in micropores the proximity between opposite walls causes the enhancement of the interactions of methane with the surfaces, which affects adsorbate packing and density. Grand canonical Monte Carlo simulations were carried out to study methane adsorption, its density under confinement, and estimate the density of the adsorbed phase in realistic overmature kerogen models that account for the chemical functionality of the surfaces and pore size distribution of kerogens of such maturity. This will provide better estimates of fluids in place. It is demonstrated that errors in the adsorbed phase have a considerable impact on gas in place calculations.