Performance and mechanism of the pyrite-kerogen complexes oxidation with H2O2 at low temperature during shale stimulation: An experimental and modeling study

Abstract

Pyrite and organic matter (OM) paly important role during rock chemical weathering and shale gas exploitation, the oxidation of which by O2 is widely recognized. But the performance and mechanism of pyrite and kerogen oxidizing with H2O2 during shale gas production are not well understood, and the impact of carbonates and clays on the oxidation process is still not known. In this study, the performance of the gas bearing carbonate-rich shale oxidation with 15 wt% H2O2 at low temperature was firstly explored. The total oxidation-dissolution capacity of the studied shale with H2O2 at room temperature was less than 5%, which is significantly poorer than shale richness in silicates. To unravel the underlying mechanisms leading to the divergent oxidation dissolution of shale with different compositions, pyrite-kerogen complexes were isolated from shale and batch experiments between individual key components including pyrite/kerogen/carbonates/clays and H2O2 were performed. On the basis of mechanistic understanding, kinetic models of the pure components reaction with H2O2 were also developed. Results show that pyrite is preferentially oxidized by H2O2 when compared to kerogen, and possesses a reaction rate 1–3 orders of magnitude larger than kerogen at different pyrite/kerogen mass ratio. Pyrite can induce the Fenton-like reactions to promote kerogen oxidation. The contribution of hydroxyl radicals on the oxidation of kerogen was about 22% for pyrite-kerogen complex with a pyrite/kerogen mass ratio of 1.65. Existence of calcite can buffer the solution pH to a circumneutral condition and induce the precipitation of Fe(III)-(oxy)hydroxides and anhydrite on the surface of the pyrite-kerogen complexes, thus to limit the generation of hydroxyl radicals and encapsulate the pyrite-kerogen complex, so as to inhibit the oxidation of these reduced components. This finding presents new insight into the mechanisms of H2O2-based oxidation stimulation of shale and provides guidance for the optimization of fracturing recipe.