Comparative study of methane adsorption of Middle-Upper Ordovician marine shales in the western Ordos Basin, NW China: Insights into impacts of moisture on thermodynamics and kinetics of adsorption

Impacts of water molecules on subsurface shale gas adsorption is a challenging topic in the assessment of shale gas resources and the selection of optimized exploitation plans. And scholars have recognized that water molecules occupy adsorption sites on hydrophilic pore walls. However, the impacts of water molecules on the thermodynamics and kinetics of methane adsorption in shales remains poorly understood. We document a series of high-pressure methane adsorption isotherms of dry and moisture-equilibrated samples of the Middle-Upper Ordovician Wulalike shale at 20-120 degrees C and up to 30 MPa. Quantitative analyses of the thermodynamic and kinetic characteristics are carried out. Results show that water molecules do not change the impacts of temperature and pressure on the trend of isotherms, thermodynamics, and kinetics of methane adsorption. The impacts of temperature, pressure, methane adsorption capacity, and water molecules on the methane adsorption are basically functioned by modifying the interactions between the adsorbate and the adsorbent. The presence of water molecules in shales leads to a decrease of similar to 77.06% in the capacity of methane adsorption, an increase of similar to 70.46% in the adsorption heat (Q(st)), a decrease of similar to 96.62% in the standard entropy (Delta S-0), and a decrease of similar to 1/9 to 1/5 in the Bangham adsorption rate (k(b)). The impacts of water molecules on the decline of methane adsorption at high pressure is lower than that at low pressure, however, on the decrease of methane adsorption rate higher than that at low pressure. The study also reflects the methane adsorption into the three-dimensional network system of kerogen. This work can improve our understanding of subsurface supercritical adsorption under reservoir conditions.

Automated summary

This study investigates the impacts of water molecules on methane adsorption in shale and finds that water molecules can modify the adsorption capacity, thermodynamics, and kinetics of methane adsorption. The presence of water molecules leads to a decrease in methane adsorption capacity, an increase in adsorption heat, a decrease in standard entropy, and a decrease in adsorption rate. This study also provides insights into methane adsorption in the three-dimensional network system of organic matter.