Abstract
Shale-persulfate interaction is ubiquitous in hydraulic fracturing, but the underlying geochemical reaction and its impact on the shale matrix are still not well understood. In this study, key components, including pyrite, calcite, chlorite, and kerogen, were chosen to react with sodium persulfate (Na2S2O8) at reservoir temperature. Results showed the oxidation-dominated reaction in the pyrite-persulfate system turned to acidification in calcite/chlorite-persulfate. In the pyrite-rich kerogen (PRK), however, kerogen competed with pyrite for persulfate consumption. An increased concentration of Na2S2O8 enhanced the dissolution of pyrite, PRK, and chlorite but resulted in the mass increase of solid in the calcite-Na2S2O8 system because of gypsum precipitation. The concentration of dissolved cations significantly correlated with the mass change of the corresponding components during persulfate stimulation, demonstrating the feasibility of water chemistry for evaluating the potential matrix alteration. Reaction in the treatment of persulfate on a multi-component mixture further suggested a complex interaction. The higher amounts of PRK, calcite, and chlorite led to the enhanced dissolution of Fe, Ca, and Mg, respectively. Various chemical reactions also influenced the dissolution behavior of cations. At low pyrite dosage, chlorite was acidly etched and slightly contributed to the dissolution of Fe. The dissolution of calcite and precipitation of gypsum affected the behavior of Ca. Element Mg mainly came from chlorite, but persulfate hydrolysis and PRK oxidation also changed its dissolution. Based on the total mass change of the multi-component mixture after persulfate treatment, a binary model was established to predict the mass change and quantify the potential matrix alteration during shale-persulfate interaction. This model could be used for selecting a potential reservoir during the in situ application of persulfate stimulation for shale permeability enhancement.
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