Acidic and Oxidation Reactions of Marcellus Shale with Na2S2O8

Abstract To evaluate the effectiveness of the acidic and oxidation reactions of stimulating organic-rich shale during fracturing, we conducted imbibition experiments using the oxidant (Na2S2O8), under no confining pressure conditions. The chemical-induced fracturing process of Na2S2O8 with Marcellus shale samples was tested and evaluated by these methods: the mass changes before and after imbibition tests were measured; mineral phase alternation and total organic carbon (TOC) before and after Na2S2O8 aqueous solutions treatment were measured, and the initiation and propagation of induced fractures, the bubble density and pH changes of the Na2S2O8 solutions were monitored. Results show that pervasive macro-fractures formed in response to acidic and oxidative dissolutions. The decrease of calcite, illite, pyrite, and TOC content after different concentrations of Na2S2O8 treatments verified that these components reacted with Na2S2O8 aqueous solutions. Na2S2O8 reacted with calcite (CaCO3), dolomite (CaMg(CO3)2 and illite through “acidic dissolution”. Besides, Na2S2O8 reacted with OM and pyrite through oxidation to form CO2 as well. The combined effect of acidic and oxidation reactions caused the fracture initiation and propagation, where the acidic reaction of calcite was dominant while oxidation reactions were secondary. Gypsum (CaSO4⸱2H2O) crystallization was formed after Na2S2O8 imbibition and the crystallization-induced stress was one of the favorable reasons to initiate the chemical-induced fractures. Compared with HCl, Na2S2O8 has improvements on the chemical-induced fracturing of Marcellus shale samples. The results indicate that adding Na2S2O8 with a proper concentration in hydraulic fracturing fluids has favorable potential to enhance the recovery of oil and gas for organic-rich shale through initiating chemical-induced fractures.