Preliminary results on the formation of organosulfur compounds in sulfate-rich petroleum reservoirs submitted to steam injection

Thermochemical sulfate reduction (TSR) is usually associated with significant sour gas (H2S) generation and calcite precipitation in deep and hot carbonate reservoirs as a result of reaction between sulfate minerals (primarily anhydrite) and petroleum fluids at elevated temperature (beginning at ∼110-140 °C). Thermal enhanced oil recovery (TEOR) techniques are widely applied during production to increase heavy oil recovery in mature fields by lowering oil viscosity. The reservoir rock may reach high temperature (150 °C < T < 300 °C) during steam and hot water flooding for secondary oil recovery that induces TSR. Experimental (pyrolysis in gold tubes) and numerical (0D fluid-rock interaction numerical model: ArXim) simulations were undertaken with a simple petroleum system assimilated to n-alkane model compounds in the presence of sulfate to assess the feasibility of “induced TSR” and to tentatively estimate the sulfur risk in terms of H2S and organosulfur compound (OSC) formation during steam and hot water stimulated enhanced recovery operations. The oxidation of organic matter (by reaction with both S0 and sulfate) can be easily reproduced under laboratory conditions, as confirmed by the presence of organic sulfur in the soluble and insoluble residue. For the experiments performed in the presence of sulfate, higher amounts of H2S, polar compounds and organic residue are produced, and aromatic fractions are enriched in condensed aromatics (sulfur-containing and non-sulfur-containing). Although sulfate reduction from 3% to 5% is deduced from a sulfur balance calculation, no secondary carbonate is detected using X-ray diffraction (XRD). Numerical simulations suggest no limiting thermodynamic factor for the precipitation of carbonate under our experimental conditions and indicate the replacement of sulfate by carbonate. According to numerical data, the lack of carbonate can be explained by its slow growth and/or high energy barrier of nucleation and/or insufficient residence time in laboratory experiments.