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New perspectives on supercritical methane adsorption in shales and associated thermodynamics
Xu Tang,Nino Ripepi,Sean Rigby,Robert Mokaya,Ellen Gilliland 한국공업화학회 2019 Journal of Industrial and Engineering Chemistry Vol.78 No.-
Understanding methane adsorption behavior in shales is fundamental for optimizing shale gasdevelopment as the adsorbed methane is a large portion of the subsurface shale gas resource. However,the adsorption mechanism of supercritical methane in shales and associated thermodynamics are poorlyunderstood because the equation of state of the adsorbed methane is unmeasurable. This work analyzedadsorption equilibria (up to 32 MPa and 393.15 K) using a rigorous framework that can account for nonidealgas properties and accurately extrapolate absolute adsorption uptakes from measured adsorptionisotherms. The framework also allowsy a straightforward calculation of thermodynamic potentialsrelevant to adsorption such as enthalpy and entropy. Modelling results show that methane adsorptionisotherms in shale under different pressures and temperatures are represented by a two dimensionaladsorption isotherm surface. The density of the adsorbed methane in shales depends on temperature andpressure, which is always lower than the liquid methane density but higher than the correspondinggaseous methane density. The temperature-dependent and pressure-dependent characteristics ofadsorbed methane density leads to the corresponding temperature-dependent and pressure-dependentmeasured/absolute adsorption isotherms. The maximum adsorption uptake of shales is independent oftemperature and pressure. The isosteric enthalpy/entropy of adsorption and enthalpy/entropy ofadsorbed methane are found to be temperature- and surface coverage-dependent. These newfindingstherefore not only clarify some historical misunderstandings of methane adsorption in shales forengineering application, but also provide a novel framework for interpreting methane adsorptionbehavior in shales and for determining the associated thermodynamics.