(247aa) Microscopic Diffusion of CO2 in Clay Nanopores

Natural gas production from shale in recent years has made a significant impact on the energy sector. It drastically changed the energy mix of the United States, making it much less reliant on coal, and hence helping its transition into cleaner energy sources. At the same, it also presents potential opportunities to mitigate global warming by using shale reservoirs for CO₂ sequestration or using CO₂for enhanced natural gas recovery.

Transport and fluids flow in conventional rocks is considered a mature subject, however, the shale’s nonporous nature challenges the common understanding of fluids and rocks. Many of the commonly used models, such as Darcy law, are no longer applicable due to the heterogeneous pore structure, which ranges from micro to mesopores. Shale is composed of two main media, organic and inorganic. In the past decade, many studies have been dedicated to the investigation of the organic part of coals as part of enhanced coal bed methane recovery, which has some resemblance to the organic part of shale. These studies were used as a basis to further study and elucidate the role of the organic matter in shale. However, there is still distinction between coal and shale and much remains not understood. Less attention has been paid to the clay part of shales. Illite, the dominant clay material in shales, has not been the subject of many scientific articles.

Molecular simulation techniques have proven to be a powerful tool in in many fields, including materials science, biology, and physics. However, they have yet to be widely utilized in the oil and gas community. Physical understanding of shale at the molecular level is critical to maximizing the value of shale. This work focuses on using equilibrium molecular dynamics to study the impact of the microporous inorganic matter of shale on CO₂ diffusion. An illite model based on ab initiocalculations is used to represent the clay matrix of shale. The canonical ensemble (NVT) was used to carry out simulations with ClayFF as the force field of choice. The Einstein relation based on a random-walk movement was used to determine diffusion coefficient. Simulations were run at a temperature range of 300-500K, which resembles subsurface conditions. Pore sizes in the 10-30A range were tested. Results show that confinement has a profound impact on diffusion. The diffusion coefficient was also found to be anisotropic due to the nature of the clay surface.