Methane Adsorption Equilibrium on Marine Gas-Bearing Shale Matrices: The Polanyi Potential Theory Approach

To reveal the feasibility of the Polanyi potential theory toward methane (CH4) adsorption equilibrium on marine gas-bearing shale matrices, a quantitative analysis method was proposed to elaborate adsorption mechanisms on CH4-shale systems based on CH4 adsorption data. Additionally, the potential theory was employed to generate CH4 adsorption isotherms on shale matrices at a given temperature and further distinguish organic matter and clay mineral dependences of CH4 adsorption. Ultimately, the potential theory was used to plot the curves of CH4 adsorption capacity versus various burial depths of shale reservoirs. Results indicate that CH4 adsorption on marine gas-bearing shale matrices typically follows micropore filling. Hence, the supercritical Dubinin-Astakhov (D-A) model derived from the potential theory is capable of CH4 adsorptio n equilibrium on shale matrices. The adsorption characteristic curve via combining the Polanyi potential theory and the supercritical D-A model well coincides within the entire adsorption temperature range. Therefore, the adsorption characteristic curve is competent to predict CH4 adsorption isotherms at a given temperature. Moreover, the Polanyi potential theory approach suggests that the organic matter content of shale matrices shows a slightly positive correlation with its adsorption capability for CH4. The clay minerals exert a minor impact on the CH4 adsorption capability of the shale matrices with high organic matter contents. Instead, the contribution of clay minerals to the shale matrices with low TOC cannot be neglected. Finally, the potential theory is capable of plotting the variation curves of the CH4 adsorption capacity versus burial depth. The Polanyi potential theory can well estimate the CH4 adsorption capability of shale matrices with various burial depths, thereby benefiting shale gas assessment and production.