Geochemical factors affecting oxidation dissolution and permeability enhancement of Yichang gas-producing shale in Hubei Province

Objective Chemical oxidation and infiltration technology has become a mature method for shale gas exploitation. This study aims to investigate the influence of geochemical factors on the exploitation process. Methods In this study, the Doushantuo Formation shale in Yichang, Hubei Province, which exhibits significant potential for shale gas exploitation and contains a high organic matter content along with pyrite, was selected as the research object. Two commonly used oxidants, 15% H2O2 and 0.5 mol/L Na2S2O8, were selected for oxidation corrosion experiments conducted at normal temperature and pressure. Post-reaction measurements included cation concentration, pH, Eh, mass loss and XRD analysis. Results The results show that Na2S2O8 outperforms H2O2, and the advanced oxidation process driven by pyrite and the erosion effect of carbonate under acid production make the acidic environment more suitable for the dissolution of carbonate-rich shale. An increase in temperature enhances the thermal decomposition of Na2S2O8 and H2O2. The thermal decomposition of H2O2 produces O2, and Na2S2O8 produces H2SO4, O2 and SO4- with strong oxidation. Thus, the temperature's enhancement effect on Na2S2O8 is greater than that on H2O2. Sodium ions (chloride) can promote the release of Ca2+ and Mg2+ during Na2S2O8 and H2O2 oxidation, indicating that a high concentration of NaCl in groundwater plays a positive role in the oxidation and dissolution of shale. Both calcium ions and sulfate ions can cause gypsum precipitation by influencing the interactions between the reactions, thus affecting the oxidative dissolution of shale. Low concentrations of exogenous calcium ions reduce the buffering effect of carbonate and promote its dissolution, while high concentrations of exogenous calcium ions cause secondary mineral precipitation, block shale pores and hinder the oxidation and dissolution of shale. However, the effect of sulfate ions on the oxidative corrosion of shale differs from that of calcium ions. A low concentration of exogenous sulfate ions inhibits the oxidative corrosion of shale by generating secondary ore and impeding the oxidation of pyrite. Conclusion Therefore, in the future process of shale gas oxidative fracturing for extraction, it is necessary to preassess the water chemical parameters of the exploitation formation in advance and select the oxidation liquid and mining modes based on the temperature, pH and cationions.