Revisiting methane absolute adsorption in organic nanopores from molecular simulation and Ono-Kondo lattice model

Accurate characterization of methane absolute adsorption in shale plays an important role in estimation of gas-in-place and prediction of well productivity. Previously, methane adsorption in shale nanopores was considered as a single-layer structure. However, it has been shown that due to strong fluid-surface interactions, methane can form transition zone between the first adsorption layer and free gas phase. Such transition zone can negatively affect the accuracy of absolute adsorption estimation from excess adsorption, which is the mostly measured adsorption property in experiments. In this work, we use grand canonical Monte Carlo (GCMC) simulations to characterize the transition zone and propose a modified adsorption model. Based on the modified adsorption model, which can explicitly take into account the effect of transition zone, we use Ono-Kondo (OK) lattice model with multilayer structure to calculate the absolute adsorption in each layer and compare with GCMC simulations. The newly proposed OK model with multilayer structure only needs layer width as an input and calculates the density in each layer and subsequently the absolute adsorption by fitting the excess adsorption. While OK model can significantly reduce calculation time, discrepancy from GCMC simulation can be less than 6%. Our work should provide important insights into the accurate characterization of methane absolute adsorption from experimental measurement.