Experimental investigation on hydrocarbon generation of organic-rich shale with low maturity in sub- and supercritical water

Sub- and supercritical water, as solvents for organic matters, are promising for hydrocarbon generation of low maturity organic-rich shale. However, there is still no noteworthy study on the difference of dynamic hydrocarbon generation of low maturity organic-rich shale between sub- and supercritical water conditions. In this study, a non-isothermal heating reactor was used to simulate the hydrocarbon generation experiment of low maturity organic-rich shale at 360 °C, 21 MPa subcritical water and 400 °C, 25 MPa supercritical water within the range of 2–12 h. The differences of the hydrocarbon generation performance, product distribution, mineral composition and pore size distribution evolution between sub- and supercritical water condition were investigated. The results showed that the utilization degree of organic carbon and gas yield under supercritical water conditions were higher than that under subcritical water conditions at any given time within 2–12 h. The supercritical water condition had the advantage of an oil yield before 10 h, which was surpassed by subcritical water condition after 10 h. The relative content of saturated hydrocarbon and aromatic hydrocarbon were higher than resin and asphaltene under both sub- and supercritical water condition. The oil produced under subcritical water condition had more aromatic hydrocarbons. The selectivity of supercritical water for H₂, CH₄, C₂H₄ and C₂H₆ was higher than that of subcritical water, and the selectivity of subcritical water for CO₂ was higher than that of supercritical water within the range of 0–12 h. The pyrite removing and pore expansion effects of supercritical water were greater than subcritical water. The mechanism of supercritical water to enhance the utilization degree of organic carbon mainly includes the enhancement of thermal efficiency, solubility, diffusion in terms of physical property effect, and the promotion of free radical reaction in terms of chemical property effect. We hope that the results will provide technical and theoretical support for in-situ hydrocarbon generation of low-mature organic-rich shale.