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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.

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Carbon dioxide (CO 2 ) injection into shale enables enhanced gas recovery (EGR) associated with geological CO 2 sequestration. Although primary research has been conducted on the CO 2 -based EGR technique in shale, the factors that influence the EGR efficiency remain unclear and need to be examined. This study presents a novel nuclear magnetic resonance (NMR)-based methodology to measure the EGR efficiency caused by CO 2 injection into shale samples with various properties. Accordingly, the effects of shale properties on the CO 2 -based EGR efficiency were revealed, and a calculation model for estimating the EGR efficiency was established. The results indicated that CO 2 injection enables significant production enhancement of shale gas, with the EGR efficiency averaging 23.54% and ranging from 16.22 to 34.34%, thus indicating that the EGR efficiency varies with the shale properties. The results also indicated that a higher CO 2 -based EGR efficiency, while adhering to a higher CO 2 -sequestration capacity, usually occurs in shales with a higher content of total organic carbon, higher methane-adsorption capacity, lower permeability, and lower clay mineral content. Moreover, an estimation model is developed to forecast the CO 2 -based EGR efficiency according to the shale properties. In general, these far-reaching results are of significance for predicting the benefit of CO 2 utilization in different shale reservoirs.

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