Shale oil and gas reserves are abundant enough to meet the growing demand for energy, but the exploitation of organic-rich shale with low maturity is still a challenging work due to its high kerogen content. As both a heat carrier and an organic solvent, supercritical water has been found to be an excellent working medium for hydrogen production by biomass or coal gasification. This study is an initial attempt to determine the candidacy of organic-rich shale as a feedstock for hydrogen-rich gas generation by supercritical water gasification. The effects of temperature (500–700 °C), pressure (22–28 MPa), time (0–12 h), water/shale mass ratio (1:1–10:1), and shale particle size (5–150 mesh) were investigated in a batch reactor. The results showed that the gas products were mainly consisted of hydrogen, carbon dioxide, and methane, which were produced by the reactions of steam reforming, water–gas shift, methanation, and carbonate hydrolysis. The abundant inorganic minerals in the shale, especially carbonate, could act as the catalyst for gasification reactions and contribute a lot to carbon dioxide formation. It was found that temperature and time were dominant factors to gas yield and selectivity. Increasing the temperature promoted the endothermic reactions of steam reforming and pyrolysis and accelerated the water–gas shift reaction. Pressure increase has a less negative but negligible effect on gasification. The carbon gasification efficiency and hydrogen selectivity all first increased and then stabilized when the reaction time was prolonged, and the water–shale mass ratio was increased and (or) the shale particle size was decreased. Overall, the suggested conditions were a temperature of 700 °C, a pressure of 22.1 MPa, a water/shale mass ratio of 5:1, a time of 4 h, and the particle size range of 10–20 mesh.

Heavy oil accounts for two-thirds of the world oil resources but contributes only one-seventh of the world oil production due to its high oil viscosity and heavy distillates. Steam injection has been widely used for heavy oil recovery by heating up the reservoir to reduce oil viscosity. However, severe carbon loss to coking causes low recovery efficiency and high energy consumption. Here, we report supercritical water injection for heavy oil recovery. Supercritical water is expected to be both a heat carrier and an organic solvent, thereby not only reducing oil viscosity but also dissolving heavy distillates to avoid coking. To test its feasibility, core experiments were first conducted to simulate the recovery process. Results showed that supercritical water flooding improved oil recovery by 17% and reduced heat consumption by 34% versus classsical steam flooding. Further, to clarify its recovery mechanism, a visualization technique and a quantitative method were developed for regulating phase behaviors and upgrading reactions between heavy oil and supercritical water. Results showed that supercritical water has good miscibility with heavy oil, and it is the key to both enhanced oil recovery and in situ upgrading. High miscibility means formation of supercritical water clusters around organic macromolecules, which makes asphaltene difficult to aggregate and polymerize to form coke but easy to decompose to form maltene and recover. Overall, supercritical water injection has made great advances in enhanced oil recovery, energy saving, and in situ upgrading for heavy oil recovery. The work provides a sound basis for its application in oilfields.

Exploitation of deep extra heavy oil is a challenging work due to its high viscosity and high reservoir pressure. Supercritical water is first proposed as an injection agent, considering its favorable physiochemical properties. A novel flooding experimental system with a design temperature up to 450 °C and pressure up to 30 MPa was developed to demonstrate the feasibility of supercritical water flooding (SCWF) technology. A sand pack core with an adiabatic boundary was used to eliminate heat unbalance. The experimental results indicated that SCWF is a promising enhanced oil recovery technology. SCWF could significantly enhance oil recovery when compared with steam flooding and hot water flooding and reduce the oil viscosity simultaneously. SCWF at 25 MPa and 400 °C raised the recovery efficiency to 97.07% and reduced oil viscosity by 36.9%. The mechanism is attributed to the extraction heavy oil components into the water-rich phase by supercritical water and the formation of miscible flooding.

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