Effect of CO2 and H2O on the behavior of shale gas confined inside calcite [104] slit-like nanopore: a molecular dynamics simulation study

摘要

The effect of CO2 and H2O on the behavior of shale gas confined in calcite [104] slit-like nanopore is investigated using molecular dynamics simulation technique. The study is relevant as the advancement of enhance gas recovery (EGR) technologies requires in-depth atomistic understanding of the hydrocarbons, water, carbon dioxide, and other fracturing fluid constituents inside the nanopores of shale gas reservoirs. We are considering carbon dioxide (CO2) because it is an attractive displacing agent for enhanced gas recovery and has the potential to be an "exotic" fracturing fluid. Water (H2O) is considered as it is the major component of water-based fracturing fluids. The structural and dynamical properties of the confined species are computed. The results indicate that the presence of CO2 and H2O in the nanopore drastically affects the behavior of shale gas. The shale gas molecules that were tightly packed near the pore wall displaced towards the center by CO2 and H2O molecules. A new layer of carbon dioxide and water is formed near the pore wall. Further investigation reveals that CO2 molecules align themselves flat near the surface, whereas H2O molecules have directional orientation with oxygen atoms of water molecules pointing towards to wall. The predicted lateral (in-plane) self-diffusion coefficient values of methane, ethane, carbon dioxide and water indicate complex dynamics inside the pore. The investigation shows the fastest dynamics for methane gas followed by ethane. Both CO2 and H2O are almost immobile. The increase in temperature in the range from 300 to 450 K does not appear to have any significant impact on the behavior of the molecules inside the pore. The adsorption energies show that both CO2 and H2O have stronger interactions with calcite [104] surface than shale gas molecules.