Deformation and Swelling of Kerogen Matrix in Light Hydrocarbons and Carbon Dioxide

Shale gas has become an attractive alternative to conventional fossil fuels due to its clean-burning characteristics. Fluid molecules reside in micropores and mesopores in shale formations. Molecular simulations provide insights into the kerogen structures, adsorption of hydrocarbons and carbon dioxide, and kerogen swelling. In this work, a new approach is introduced to create atomistic configurations of kerogen matrix with specific porosity. A dummy particle is used to control the porosity. Once the kerogen matrix is created, the dummy particle is removed and a limited number of nails are placed on the periphery of the pores to prevent the kerogen matrix from collapsing and to keep its basic structure intact. The hybrid Molecular Dynamics-Grand Canonical Monte Carlo (MD-GCMC) simulations are performed to investigate the adsorption and kerogen swelling of five hydrocarbon gases (methane, ethane, propane, n-butane, and i-butane at 393.15 K and various pressures), n-pentane liquid (at 298.15 K and 1 atm), and supercritical carbon dioxide (at 393.15 K to a pressure of 400 atm). The kerogen matrix is flexible. Our simulation results show that there is a coupling between adsorbate molecular size and shape and deformation of the kerogen matrix structure. The flexibility of the kerogen matrix affects swelling. The kerogen matrix deformation allows small adsorbate molecules to dissolve in the matrix. Our simulations results show that kerogen swelling decreases with the increase of the molecular size of the adsorbate (CO₂ > CH₄ > C₂H₆ > C₃H₈). The deformation and increase in swelling by n-pentane is much more pronounced than by methane and other light hydrocarbon gases. Our work is based on type II-A kerogen macromolecules. The methodology can be applied to other types of kerogen molecules.