Experimental study on the oxidative dissolution of carbonate-rich shale and silicate-rich shale with H2O2, Na2S2O8 and NaClO: Implication to the shale gas recovery with oxidation stimulation

Abstract: Oxidation stimulation of gas-bearing shale is promising in enhancing shale gas recovery by improving shale matrix conductivity and promoting gas desorption. However, this technology is still in its infancy. Shale-oxidant interaction which is critical for the efficiency, safety and environment friendly of shale oxidation stimulation is still needed to be explored. In this study, carbonate-rich shale and silicate-rich shale, collected from Shuijingtuo Formation in Yichang, Hubei province of China, were used to conduct oxidative dissolution experiments with hydrogen peroxide (H2O2), sodium hypochlorite (NaClO) and sodium persulfate (Na2S2O8) at formation temperature. Combined with the analysis of water chemistry of reacted solutions, as well as the mineral composition, micromorphology, TOC content and mass change of shales, the conclusion that silicate-rich shale is more suitable for oxidation stimulation can be obtained. Na2S2O8 is more superior than H2O2 and NaClO, while the optimal OM removal oxidant is NaClO. The contribution of mineral-oxidants reaction is more profound than the organic matter removal to shale oxidative dissolution. Carbonates and pyrite are both critical minerals. Carbonates dissolution can buffer the solution pH which impedes the acid corrosion of other minerals and also releases Ca to participate in the precipitation of gypsum, which brings threaten to the effectiveness of shale oxidative stimulation by clogging pores throat and fracture or may help to induce spontaneous microfracture propagation caused by crystal growth. What’s more, the dissolution of carbonates can accelerate the oxidation of organic matter, however the precipitation of gypsum can inversely hinder its oxidation. Iron-bearing mineral precipitation and other aluminosilicate minerals precipitation at alkaline condition is another important negative factor for shale oxidation stimulation. In order to accurately evaluate the effect of oxidation stimulation on shale gas recovery, the flow-through experiments should be conducted in the future.