Carbon Dioxide Sequestration and Hydrocarbons Recovery in the Gas Rich Shales: An Insight from the Molecular Dynamics Simulations

Recent technological advances like horizontal drilling and hydraulic fracturing have made recovery of gas possible from the ultra low permeable shale plays in the United States. However, recoveries from these gas shales still tend to be in the range of single digits. It is believed that more than 80% of the generated hydrocarbons still remain in these tight formations. In order to increase the recovery from these tight shale plays, an enhanced recovery procedure using supercritical carbon dioxide as the injection fluid is recommended in this paper. In the current research, molecular dynamics simulations (MDS) have been performed on the kerogen-methane-carbon dioxide system to understand the absorption-adsorption-desorption phenomena of the super critical carbon dioxide fluid.

Previous studies have confirmed that the kerogen has a tendency to adsorb and absorb the hydrocarbons.  In the current work, the type II kerogen model was chosen and annealed with the methane molecules. The Nose-Hoover style non-Hamiltonian equations of motion were used in a molecular simulator to generate positions and velocities sampled from the canonical (nvt) and isothermal-isobaric (npt) ensembles. This updates the position and velocity for atoms in the group each time step during the simulations. The pairwise distribution function and density of the mixture were calculated at the end of the simulations in order to validate the model with the experimental observations. The subsequent simulations with the carbon dioxide molecules in the periodic boundary conditions reveal that the carbon dioxide can sweep the absorbed methane from the kerogen matrix in the shales. The interactions at the interface between the carbon dioxide and the methane rich kerogen matrix are studied. These replicate the interactions at the fracture surface in the process of using super critical carbon dioxide as the fracing fluid for enhanced gas recovery. Compared to methane, the carbon dioxide molecule has higher affinity to be adsorbed and eventually absorbed in the kerogen matrix because of its polar nature and linear shape. This allows the carbon dioxide to replace methane in the kerogen, thereby, sequestrating itself in the kerogen rich shale formations. Adsorption and absorption of the carbon dioxide in turn cause desorption of the methane from the kerogen, which aids in the gas recovery from such gas rich shales as the Marcellus Shale in the United States.