Geochemical Evaluation of Compressed Air Energy Storage in Depleted Shale Gas Reservoirs

The limited distribution of suitable salt caverns has stimulated interest in depleted hydrocarbon reservoirs as alternative geological media for large-scale underground compressed air energy storage (CAES). Depleted shale gas reservoirs may offer connected fracture networks for rapid air injection and withdrawal and adjacent matrix pores for additional storage, while their relatively limited mobile water may reduce the gas–water displacement losses encountered in conventional water-driven reservoirs. However, black shale contains organic matter and reduced minerals that can react with injected O2 and deplete oxygen from stored air. This process is particularly important for fuel-assisted CAES because the recovered air serves as both the expansion working fluid and the oxidant for fuel combustion. Oxygen–shale interactions in Longmaxi Formation shale were investigated using scanning electron microscopy, repeated electron probe microanalysis (EPMA) mapping, and high-pressure batch experiments under different temperatures, moisture conditions, and successive air-exposure cycles. Organic matter and Fe(II)-bearing minerals, including framboidal pyrite and ferroan dolomite, were identified as widely dispersed, potentially oxygen-reactive constituents. PHREEQC calculations were further used to compare the thermodynamic oxygen-consumption potentials of pyrite, ferroan dolomite, and chlorite under experimental and reservoir-relevant temperature–pressure conditions. Repeated EPMA mapping showed C loss and O enrichment in organic matter and Fe–S loss accompanied by O enrichment in pyrite, with the strongest cumulative alterations occurring after subsequent moist oxidation. After 10 d under dry conditions, oxygen consumption ranged from 139.3 to 232.7 mmol kg–1 between 80 and 140 °C, with residual O2 concentrations of 13.4–16.9%. At 100 °C, water increased oxygen consumption from 139.3 to 202.9 mmol kg–1, an increase of 45.7%, and decreased the residual O2 concentration from 15.4 to 14.0%, indicating that increasing temperature and water availability intensify oxygen-shale reactions and associated oxygen depletion. CO2 was the principal carbon-bearing gaseous product, but it accounted for only part of the consumed oxygen. Oxygen consumption per exposure declined by approximately 72% between the first and third cycles, indicating depletion or passivation of readily accessible reactive sites. Thermodynamic modeling further indicated that pyrite dominated the calculated oxygen-consumption potential of the three modeled Fe-bearing minerals, contributing approximately 86%, while equilibrium residual O2 was strongly influenced by pressure-dependent changes in the gas-to-solid ratio. These results identify oxygen depletion as an important geochemical constraint on fuel-assisted CAES in depleted shale gas reservoirs and demonstrate that temperature, water availability, air-rock contact time, exposure history, and gas-to-solid ratio should be incorporated into reservoir-scale assessments of produced-air quality and supplementary-combustion performance.