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 ¹³C 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 (PFG) NMR demonstrate that the self-diffusion coefficient of CO₂ is an order of magnitude smaller than that of CH₄, and the existence of CO₂ slows down the diffusion of CH₄ slightly. The gas diffusion in both scenarios follows the trend: SBA-15 > SBA-phenyl > SBA-cyclohexyl.