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.