Investigation of supercritical shale gas adsorption in shale based on the Ono-Kondo lattice model

Shale formations are typically buried deeper with higher temperature and pressure than conventional reservoirs,resulting in a significant proportion of shale gas being adsorbed under the supercritical state.Because the mechanism of supercritical adsorption is substantially different from that of conventional subcritical adsorption,there are still some challenges in applying the adsorption model established in subcritical conditions to the field of supercritical shale gas adsorption.Consequently,an accurate representation of supercritical shale gas adsorption is a prerequisite for the accurate assessment of gas in place and formulation of reasonable development plans.To address the aforementioned issues of supercritical CH4 adsorption in shale,the adsorption isotherms and bulk densities of supercritical CH4 at various temperatures (40,60,80 and 100 ℃) were determined.The gravimetric adsorption experiments used the gravimetric apparatus equipped with a high-precision magnetic suspension balance.The results indicate that the gravimetric method can simultaneously determine the bulk methane density and the excess adsorption.The excess adsorption increases with pressure and then decreases; the excess adsorption decreases with increasing temperature,and the pressure corresponding to the maximum excess adsorption increases with increasing temperature.The Ono-Kondo lattice model was used to analyze monolayer and multilayer adsorption without any assumptions about the density and volume of the adsorbed phase.The results of the Ono-Kondo lattice model show that the supercritical CH4 adsorption in shale can only form monolayer,in which the adsorbed phase density and absolute adsorption volume increase monotonically with pressure,and the adsorbed phase volume remains approximately constant.As the equilibrium pressure increases,the difference between the density of adsorbed phase and the density of bulk phase first increases and then declines,which can result in a maximum amount of excess adsorption.Assuming a constant value for the density of the adsorption phase would underestimate the absolute adsorption in the high-pressure range.