Study on the pathways of thermochemical sulfate reduction between sulfates and alkanes at low temperatures.

• The rate-limiting step of sulfate reactions with alkanes at low temperatures is the oxidation of SO 4 2− to SO 3 2−. • The reaction pathway of TSR reaction between SO 4 2− and CH 4 at low temperatures is SO 4 2− → SO 3 2− → HSO 2 − → HSO− → HS− → H 2 S. • The oxidation order of sulfate in low temperature TSR reaction is HSO 4 − > [MgSO 4 ] CIP >SO 4 2−. • The sequence of alkanes participating in TSR reactions is n -C 5 H 12 > n -C 4 H 10 > C 3 H 8 > C 2 H 6 > CH 4. To reduce the energy consumption of heavy oil thermal recovery, low-temperature hot water flooding is becoming popular. The occurrence of thermochemical sulfate reduction (TSR) reactions during the hot water flooding process generates H 2 S, which can corrode equipment and jeopardize personnel safety. Preventing and managing H 2 S requires studying TSR reactions. The study investigated the reactivity of SO 4 2-, HSO 4 -, and [MgSO 4 ] CIP in the TSR reaction during heavy oil low-temperature hot water flooding. The results indicated that during low-temperature hot water flooding, no significant thermal cracking of heavy oil occurs, only a minor aquathermolysis takes place, while the TSR reaction proceeds vigorously. The TSR reaction leads to increased yields of H 2 S, CH 4 , and CO 2 , as well as a higher S content in the heavy oil. Based on experimental results, numerical simulations of the reaction process between SO 4 2- and CH 4 were conducted using density functional theory and transition state theory. The results indicated that the reduction of SO 4 2- to produce SO 3 2- is the rate-limiting step in the entire reaction pathway between SO 4 2- and CH 4. Further analysis of the reaction barriers and chemical reaction rates of the three sulfates (SO 4 2-, HSO 4 -, and [MgSO 4 ] CIP) and C1-C5 alkanes was conducted based on the limiting step. Through macro and micro analysis, this study not only confirmed previous findings regarding the order of alkanes participating in TSR reactions from the perspective of microscopic molecular but also clarified the oxidation order of sulfate species under heavy oil low-temperature thermal water flooding conditions, with the sequence being HSO 4 - > [MgSO 4 ] CIP >SO 4 2-. This study provides a foundational understanding of TSR reactions during heavy oil low-temperature hot water flooding and is of significance for the prediction and management of H 2 S. [ABSTRACT FROM AUTHOR]