Sub- and supercritical water conversion of organic-rich shale with low-maturity for oil and gas generation: using Chang 7 shale as an example

Abstract

Organic-rich shale resources are large reserves with high hydrocarbon generation potential but are difficult to exploit due to their high solid kerogen content. Supercritical water conversion was proposed as an alternative method to convert kerogen into oil and gas because supercritical water has favorable solubility, dispersion, and reactivity. In this study, Chang 7 shale containing a high TOC content of 15.11%, type II kerogen, and low Ro of 0.36–0.38% in the Ordos basin was taken as a typical example of organic-rich shale with low maturity. A series of experiments at the temperatures of 300–650 °C and a pressure of 25 Mpa were carried out to test the feasibility, and the shale conversion performance was analyzed from three perspectives, hydrocarbon generation of kerogen, the effect of inorganic minerals, and shale pore evolution. The optimum oil and gas yields were, respectively, found to be 352.1 mg (g TOC)⁻¹ (g TOC refers to total organic carbon mass in shale) at 380 °C and 852.0 mL (g TOC)⁻¹ at 650 °C. Compared with pyrolysis in the aluminium retort, supercritical water conversion raised the oil yield at the same temperature (171.4 mg (g TOC)⁻¹ at 380 °C) or reduced the temperature with the same yield (346.8 mg (g TOC)⁻¹ at 520 °C). Chang 7 shale minerals as a whole increased the oil yield by 34.2% at 380 °C but had a negligible effect on gas generation. Among them, the carbonate (dolomite) promoted oil generation but inhibited gas generation, while the silicates (feldspar, quartz, and clay) did the opposite, and the pyrite favored both oil and gas generation. Additionally, the supercritical water conversion significantly increased the shale pore volume and specific surface area because the hydrocarbon generation of kerogen produced many nanopores with slit-like shapes and diameters of 50–5000 nm. This paper provides an in-depth understanding of sub- and supercritical water conversion of low-maturity shale for oil and gas production.