Mechanism Analysis of Shale Gas Adsorption under Carbon Dioxide–Moisture Conditions: A Molecular Dynamic Study

In recent decades, shale gas, which has been regarded as a source of clean energy, is gradually replacing conventional energy. Shale gas adsorption in carbon dioxide (CO2)–moisture systems has been discussed in many previous studies; however, the intrinsic mechanism has not been clarified yet. In this work, the molecular dynamic (MD) method is adopted to study the adsorption behaviors of shale gas adsorption in the realistic kerogen nanoslit. The spatial density distributions of shale gas and different components have strong inhomogeneity. To reveal the heterogeneous adsorption mechanism, the potential of mean force (PMF) distributions of shale gas components are calculated on different target positions for the first time. The water (H2O) component prefers to adsorb on the oxygen-enriched position, as a result of the strong molecular polarity and hydrogen bond interactions. The CO2 component tends to adsorb on the carbon-rich site, which is the result of combining the van der Waals interaction and molecular polarity with kerogen walls. The potential energy contours are computed to verify the affinities between different components and the kerogen surface, and the potential energy difference can be observed between the bulk phase and adsorbed phase, which corresponds to the density and PMF analyses. The sensitivity analysis is also carried out to verify the above mechanism explanation. Higher temperature facilitates the desorption of shale gas, and higher pressure leads to more adsorption quantity. In the larger pore space, because of more content of H2O and CO2 molecules, the adsorption amount of methane (CH4) decreases. More content of CO2 is conducive to the desorption of shale gas, verified by cases in various component proportions.