An experimental investigation into the oxidative dissolution of typical organic-rich shale from China

Oxidizing stimulation of organic-rich shale was proposed to enhance the rate of methane extraction from the matrix. Previous studies lacked attention on the dissolution characteristics of organic matter (OM) during the dissolution process performed using oxidative fluids. In this study, the organic-rich shale samples obtained from the Longmaxi Formation, Niutitang Formation, and Yanchang Formation were used for dissolution experiments. Crushed shale samples were treated with deionized water, hydrogen peroxide (H2O2), or sodium hypochlorite (NaClO) for 240 h, respectively. By using a combination of X-ray Diffraction, rock-eval pyrolysis, total organic carbon analysis, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy, the composition and organic geochemical characteristics of the treated shale samples were determined. The results showed that H2O2 induces the dissolution of both the pyrite and carbonate minerals, and NaClO more readily dissolves pyrite and not carbonate minerals. Higher removal efficiency of OM for all four shale samples occurs when treated with NaClO than with H2O2. The oxidative dissolution of OM in these shale samples occurs as two dominant modes: one is the oxidation cracking of C–C, C–H such as in the Niutitang or Yanchange formations; the other is oxidation cracking of C–O, such as in the Longmaxi Formation samples. Both the Longmaxi Formation samples, which showed higher C–O contents compared to samples from the other two formations, exhibited better oxidizability than others when treated with H2O2 or NaClO. Oxidation products such as Fe2+/Fe3+ and H+ enhance the oxidizing ability of H2O2 and NaClO toward cracking of OM. However, the consumption of H+ due to the carbonate minerals may impair the oxidizing ability of H2O2, but not that of NaClO. Furthermore, the presence of an organo-clay complex has an adverse influence on the removal of OM. The results indicated that the type of OM, the content of the pyrite and carbonated minerals, and the physical protection of the organic carbon by clays should be taken into account, when selecting a shale reservoir suitable for oxidizing stimulation and associated optimization of the oxidant during the fracturing operation design.