Effect of hydrocarbon type on thermochemical sulfate reduction

Abstract

Thermochemical sulfate reduction (TSR) of model compounds and whole crude oils was investigated to determine the influence of hydrocarbon type on the reaction. A series of isothermal gold-tube hydrous pyrolysis experiments under a constant confining pressure of 24.1 MPa (3500 psi) was conducted where individual organic compounds containing different functional groups (sulfur free) were reacted in the presence of MgSO 4 and the amounts of generated hydrogen sulfide were measured. As MgSO 4 is the only sulfur source initially present in these experiments, any hydrogen sulfide formed must arise from TSR. The model compound experiments show that the type of hydrocarbon involved can significantly affect the extent of TSR reaction, and the difference in hydrocarbon reactivity involved in sulfate reduction mainly depends on the type of functional group present. For the model compounds studied, the relative reactivity during TSR is 1-octene > 1-octanol > 1-octanone > n-octane > octanoic acid > octylbenzene > xylene. An additional series of non-isothermal hydrous pyrolysis experiments, heated from 340 °C to 550 °C at 2 °C/h, were conducted involving MgSO 4 and three different crude oils with variable total sulfur contents ranging from 0.5% to 5.2% and with a sulfur-free paraffinic mixture. Comparison of H 2S generation as a function of temperature from these experiments suggests that crude oil composition can dramatically affect the onset temperature of TSR. Although the model compound experiments clearly demonstrate the variable reactivity of different hydrocarbon types during TSR reactions, the observed trends for sulfate reduction during the whole oil experiments indicate that the labile sulfur content in oil may be more significant in controlling TSR reaction rates than the hydrocarbon chemistry. The absence of intermediate reduced sulfur species (e.g., elemental sulfur, sulfites, etc.) in the solid residuals after TSR does not preclude the possibility that generated H 2S reacts with sulfate as these reactive sulfur species will rapidly react with the hydrocarbons.