Absolute Adsorption Calculation of Shale Gas Based on the Ono–Kondo Lattice Model

Adsorbed gas is the main occurrence state in shale gas, and its adsorption mechanism is of great significance for accurate evaluation of shale gas reserves. The adsorbed phase density is the key to accurately evaluate the adsorbed gas content. At present, there is no consensus on the adsorption mechanism of shale gas, resulting in different calculation results of adsorbed phase density, which further affects the calculation of absolute adsorption. Therefore, based on previous experimental studies on CH4 adsorption in shale at high pressure, the Ono–Kondo lattice (OK) model is used to investigate the influence of different adsorption mechanisms on the calculation of adsorbed phase density and the calculation of absolute adsorption, and the parameter range of the OK model suitable for describing the CH4 adsorption behavior in shale is determined. It is found that the modified two-layer OK model is suitable to describe the CH4 adsorption behavior in shale, and the adsorbed phase density varying with temperature and pressure can more accurately predict the CH4 absolute adsorption in shale. The CH4 adsorption mechanism in shale is two-layer adsorption. The mainstream monolayer adsorption overestimates the adsorbed phase density and underestimates the adsorbed phase volume, which leads to an underestimation of the adsorbed gas content and an overestimation of the free gas content. These conclusions are of great significance for accurate evaluation of shale gas reservoirs.