Molecular investigation of the interactions of carbon dioxide and methane with kerogen: Application in enhanced shale gas recovery

Even with technological advancements such as hydraulic fracturing and horizontal drilling, only 20% of the original gas in shales is recoverable through current industry practices. This low recovery factor is attributed to very low permeability and sorption of much of the gas by solid organic matter. Enhanced gas recovery through carbon dioxide sequestration could be performed either by cyclic injection (huff and puff) of carbon dioxide or fracturing/re-fracturing the formation with a viscosified, foamed or energized carbon dioxide hydraulic fracturing fluid. This work employs molecular modeling to conduct a fundamental investigation of the interactions between carbon dioxide and the solid organic part of the shales, the majority of which is kerogen. In the current work, more realistic molecular models for Type II kerogen in oil-gas window, with active sites were used instead of the graphite based carbon models used in many previously published works. Both adsorption onto kerogen surfaces and absorption within it contribute to its high sorption capacity. Previously, researchers have shown through modeling and experiments that kerogen has a higher affinity and adsorption capacity for carbon dioxide than for methane. The current work studies the sorption of carbon dioxide and methane using quasi equilibrium Molecular Dynamics (MD) simulations. The MD simulations of ternary system of methane, carbon dioxide and kerogen revealed that the carbon dioxide is more strongly retained than methane in the bulk kerogen matrix. The self-diffusion coefficient of carbon dioxide (Dself = 10-10 m2/s) in the kerogen was found to be an order of magnitude smaller than that of methane (Dself = 10-9 m2/s) in the kerogen. The MD simulations revealed that in the process of carbon dioxide - methane 1:1 exchange in the kerogen matrix, the kerogen matrix shrinks in volume. This may lead to disturbance in the fluid pathways that contribute to fluid flow in shales. The molecular investigation performed in this fundamental work is relevant to any carbon dioxide enhanced gas production from shale gas resources.