A New Method for Correcting Shale High-Pressure Adsorption Curves Based on Instantaneous Adsorption Capacity

Abstract

Summary A high-pressure isothermal adsorption curve is the basis of calculating the amount of gas adsorbed in a shale reservoir. Although shale gas adsorption measurements report an excess adsorption amount as a function of pressure, this leads to a widely known effect in which adsorption measurements show a decrease in the excess adsorption amount at high pressure. Therefore, it is not appropriate to use the excess adsorption amount to calculate the amount of gas adsorbed. Understanding adsorption mechanics and correcting the high-pressure adsorption curve of shale is critical for accurate assessments of adsorbed shale gas. This study uses a high-pressure (more than 50 MPa) experimental setup addressing this effect in the context of hydrocarbon gas (methane) in shales. By observing the change in adsorption amount between two pressure measurement points, the instantaneous adsorption capacity (IAC) is defined to calculate the cumulative adsorption capacity (CAC). Based on an analysis of the adsorption mechanism and measurements, a new method is proposed to correct the adsorption curves and obtain absolute adsorption values. The results show that the IAC is lower than zero when the testing pressure is higher than 15 MPa. This is because the commonly used volumetric method for calculating adsorption capacity does not consider the volume of the adsorption phase. Based on the assumption that the IAC is not less than zero, the volume of the adsorbed phase at different pressures is calculated by defining the equivalent diameter of the adsorbed molecules. The corrected results show that the calculated equivalent diameter of an adsorbed methane molecule is 0.58 to 0.65 nm. Based on the modified instantaneous adsorption curve, a high-pressure adsorption model is established to fit and predict the CAC. This model provides a simpler and more practical method for obtaining the accurate amount of shale gas adsorbed.