CO2/CH4 Competitive Adsorption in Shale: Implications for Enhancement in Gas Production and Reduction in Carbon Emissions.

CO2/CH4 interaction determines the prospects for complementary enhanced gas recovery (EGR) associated with CO2 sequestration in shale. We characterize the competitive adsorption of CO2 and CH4 in shale using low-field NMR. Competitive sorption of CO2 relative to CH4 is defined as the CO2/CH4 competitive adsorption ratio (CO2/CH4 CAR for short) when CO2 and CH4 have the same original partial pressure in shale. Results indicate the CO2/CH4 CAR decreases with the logarithm of increasing pressure. Observed CO2/CH4 CARs are on the order of 4.28-5.81 (YDN-1) to 3.43-5.57 (YDN-2), describing the remarkable competitive advantage of CO2 sorption relative to CH4 for shale. Results also indicate that increasing the CO2/CH4 pressure ratio (1) increases the adsorption capacity of shales to CO2 and decreases that to CH4 logarithmically with pressure, and (2) boosts CO2-CH4 displacement and generates greater EGR efficiency in shale, where the EGR efficiency can be inferred by the CO2/CH4 pressure ratio using a Langmuir-like function. Furthermore, the maximum sequestration capacity of adsorbed CO2 during CO2-CH4 competition is on the order of ∼3.87 cm3/g (YDN-1) to ∼5.13 cm3/g (YDN-2). These promising results for EGR and CO2 storage reveal the considerable potential for carbon capture and geological sequestration in shale.