Experimental and theoretical studies on kinetics for thermochemical sulfate reduction of oil, C2–5 and methane

Abstract To address the kinetic parameters for thermochemical sulfate reduction (TSR) of oil and hydrocarbon gases, hydrothermal experiments involving oil or methane and sulfate at 50 MPa were conducted by a gold tube pyrolysis system. By determination of gas yields in non-isothermal pyrolysis of oil with gypsum and water, it was found that the generation of H 2 S can be divided into two stages, including TSR of liquid oil and C 2-5 hydrocarbon gases. The average activation energies (E a ) for TSR of oil and C 2-5 were fitted to be 58.1 and 65.9 kcal/mol, respectively. By isothermal pyrolysis of methane-dominated gas with aqueous MgSO 4 at 500–600 °C, the reaction rates for TSR of methane at elevated temperature was determined. Meanwhile, theoretical calculations based on density functional theory (DFT) and transition state (TS) were carried out to obtain the thermodynamic parameters for the oxidation of methane by different sulfate species. It is revealed that the energy barriers for reactions between methane and HSO 4 − or [MgSO 4 ] CIP at 0–600 °C were much lower than those for free SO 4 2− . Combined the experimental results and theoretical calculations, the E a for TSR of methane involving SO 4 2− , HSO 4 − and [MgSO 4 ] CIP were calculated to be 67.0, 52.1 and 54.9 kcal/mol, respectively. Finally, the conversion for TSR of oil, ethane and methane in the subsurface were addressed by geological extrapolation and their onset temperatures in different geological conditions were proposed. It is implied that the onset temperature of methane TSR may be as higher as 200 °C when no catalysts (i.e. S, H 2 S and S 3 − ) and high concentration of MgSO 4 were presented.