Li, and 6 co-authors, reported in 2010 that the drastic decline of shale-gas production after fracturing depresses the development of this unconventional gas resource. Although shale oxidant stimulation can dissolve unstable composition to enhance permeability, the shale-fluids interaction during this stimulation process is still not yet clear. In this study, the organic-rich shale collected from the Cambrian Shuijingtuo Formation of Yichang, Hubei province, China was selected to react with three oxidants, hydrogen peroxide (H2O2), sodium hypochlorite (NaClO) and sodium persulfate (Na2S2O8) at formation temperature.
Variation of water chemistry, mineral composition and micromorphology were analyzed to reveal the mechanism of shale oxidative dissolution and evaluate its influence on shale permeability enhancement. Results showed that pyrite and OM can be discreetly oxidized with different oxidants. At the same oxidation duration, the acidic environment was beneficial for carbonate dissolution, while the alkaline environment was favorable to the dissolution of dolomite and tectosilicate minerals such as quartz, albite, illite and chlorite. Nevertheless, the serious thermal decomposition of H2O2, precipitation of gypsum and ferric hydroxide (Fe(OH)3) occurred during shale-fluids interaction at formation temperature might impede the enhancement of shale permeability. The oxidative dissolution of shale also brought about the release of trace elements, which might result in groundwater pollution. For the in-situ application of shale oxidative dissolution, difficulties such as thermal decomposition, secondary minerals precipitation and possible groundwater pollution should be considered further in the future.
The authors concluded that the shale oxidative dissolution experiments with three different oxidants, H2O2, NaClO and Na2S2O8, were carried out at formation temperature to illustrate the shale-fluids interaction mechanism and estimate its feasibility of enhancing shale permeability. The solution water chemistry, solid mineralogy and micromorphology characterization, TOC content and mass loss were analyzed and the results demonstrated that:
1) The pyrite and OM could be oxidized by all candidate oxidants, however, the oxidation efficiency was not only related to the property of the oxidants (e.g. thermal stability, pH), but also depended on the occurrence of unstable composition in shale.
2) The extra-added oxidants could result in complicated shale-fluids interaction, which leaded to the minerals dissolution and secondary minerals precipitation. The reaction was dominant by carbonate dissolution as well as gypsum precipitation in the acidic environment, while a large amount of hydroxyl in alkaline environment promotes the dissolution of dolomite and tectosilicate minerals such as quartz, albite, illite and chlorite, as well as precipitation of secondary minerals. The shale permeability variation, which might be seriously affected by the dissolution and precipitation of the minerals, still needs precise assessment in the laboratory as well as in-situ.
3) The oxidative dissolution of shale, especially OM and pyrite, would release trace elements and result in groundwater contamination. This environmental problem should be taken into consideration before the shale oxidative dissolution technology can apply for gas shale stimulation.
[The paper has significant application to uranium roll-front bio-chemical cell discussions]
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