Oxidative dissolution kinetics of organic-rich shale by hydrogen peroxide (H2O2) and its positive effects on improving fracture conductivity

Abstract

Maintaining the fracture network created by hydraulic fracturing remains challenging because unpropped fractures close during production, resulting in shale gas production decline. Chemical dissolution is being implemented to maintain unpropped fracture conductivity, and the effect of oxidative dissolution on organic-rich shale may have the potential to maintain these fractures. In this paper, the change of organic matter (OM) in crushed shale samples exposed to hydrogen peroxide (H2O2) or deionized water were characterized. Reaction kinetics experiments were run using sliced shale samples and H2O2 with a mass concentration of 2–10% at temperature of 40–80 °C. Stress sensitivity of fractured shale plugs after oxidation for 72 h was evaluated. Results show H2O2 can remove 56.87% of solid OM and 55.34% of extractable OM in the crushed sample. Total organic carbon in H2O2 filtrate after the reaction is 91.36 mg/L,while that of deionized water is 30.88 mg/L. A significant decrease of C–O according to the C1s and O1s spectra of the crushed samples after oxidation means that some of oxygen contained functional groups oxidized and cleaved. However, reaction rate of oxidative dissolution occurs slowly and significantly depends on the concentration and temperature. The relationship between reaction rate and concentration can be described as a power function, and the relationship between reaction rate and temperature can be described as an exponential function. Activation energy of oxidative dissolution ranges from 3.51 to 12.10 kJ/mol when H2O2 mass concentration ranges from 2% to 10%. Oxidative dissolution can increase fracture width judging by estimation based on the mass loss. Stress sensitivity coefficient of the plugs treated with deionized water are 0.52 and 0.59, while the plugs treated with H2O2 are 0.48 and 0.52. It indicates that oxidative fluids can play a role in maintaining unpropped fracture conductivity enhancing shale gas recovery.