Displacement and Diffusion of Methane and Carbon Dioxide in SBA-15 Studied by NMR

With increasing concern about the environmental impact of shale gas exploitation, nonaqueous fracturing with carbon dioxide has emerged as a promising alternative to increase gas production and, at the same time, to store large amounts of CO2. The key process of CH4 displacement by CO2 is worth a systematic investigation from aspects of both experiment and simulation. In this work, the CH4 and CO2 displacement was studied with in situ C-13 NMR in the pores of silica (SBA-15), which were functionalized with organic groups such as phenyl and cyclohexyl, in order to model the organic matter in shale with different aromaticity. Due to the stronger adsorption strength and higher capacity of CO2 in SBA-15, CH4 can be easily stripped out of the pores by CO2, while the reverse process to displace CO2 with CH4 is not effective. Even though the displacement effect in the pores of SBA-15 with a higher aromaticity is relatively better at room temperature, the superiority is eliminated by high temperature. Furthermore, the results of pulse field gradient (PEG) NMR demonstrate that the self-diffusion coefficient of CO2 is an order of magnitude smaller than that of CH4, and the existence of CO, slows down the diffusion of CH4 slightly. The gas diffusion in both scenarios follows the trend: SBA-15 > SBA-phenyl > SBA-cyclohexyl.