Experimental investigation on thermochemical sulfate reduction by H 2S initiation

Abstract

Hydrogen sulfide (H 2S) is known to catalyze thermochemical sulfate reduction (TSR) by hydrocarbons (HC), but the reaction mechanism remains unclear. To understand the mechanism of this catalytic reaction, a series of isothermal gold-tube hydrous pyrolysis experiments were conducted at 330 °C for 24 h under a constant confining pressure of 24.1 MPa. The reactants used were saturated HC (sulfur-free) and CaSO 4 in the presence of variable H 2S partial pressures at three different pH conditions. The experimental results showed that the in- situ pH of the aqueous solution (herein, in- situ pH refers to the calculated pH of aqueous solution under the experimental conditions) can significantly affect the rate of the TSR reaction. A substantial increase in the TSR reaction rate was recorded with a decrease in the in- situ pH value of the aqueous solution involved. A positive correlation between the rate of TSR and the initial partial pressure of H 2S occurred under acidic conditions (at pH ∼3–3.5). However, sulfate reduction at pH ∼5.0 was undetectable even at high initial H 2S concentrations. To investigate whether the reaction of H 2S (aq) and HSO 4 - occurs at pH ∼3, an additional series of isothermal hydrous pyrolysis experiments was conducted with CaSO 4 and variable H 2S partial pressures in the absence of HC at the same experimental temperature and pressure conditions. CaSO 4 reduction was not measurable in the absence of paraffin even with high H 2S pressure and acidic conditions. These experimental observations indicate that the formation of organosulfur intermediates from H 2S reacting with hydrocarbons may play a significant role in sulfate reduction under our experimental conditions rather than the formation of elemental sulfur from H 2S reacting with sulfate as has been suggested previously (Toland W. G. (1960) Oxidation of organic compounds with aqueous sulphate. J. Am. Chem. Soc. 82, 1911–1916). Quantification of labile organosulfur compounds (LSC), such as thiols and sulfides, was performed on the products of the reaction of H 2S and HC from a series of gold-tube non-isothermal hydrous pyrolysis experiments conducted at about pH 3 from 300 to 370 °C and a 0.1-°C/h heating rate. Incorporation of sulfur into HC resulted in an appreciable amount of thiol and sulfide formation. The rate of LSC formation positively correlated with the initial H 2S pressure. Thus, we propose that the LSC produced from H 2S reaction with HC are most likely the reactive intermediates for H 2S initiation of sulfate reduction. We further propose a three-step reaction scheme of sulfate reduction by HC under reservoir conditions, and discuss the geological implications of our experimental findings with regard to the effect of formation water and oil chemistry, in particular LSC content.