Rates and Mechanisms of Thermochemical Sulfate Reduction: Implications for Model Predictions of H2S Risk
Although the process of thermochemical sulfate reduction (TSR) has been recognized by geochemists for over 50 years, it has proven extremely difficult to simulate in the laboratory under conditions close to those encountered in geologic settings. Published estimates of the kinetic parameters that describe the rate of the TSR reaction vary widely and are often inconsistent with geologic observations. Consequently, the prediction of the hydrogen sulfide (H2S) risk for a given reservoir prior to drilling remains a major challenge for the oil industry. Recent experimental and theoretical discoveries indicate that magnesium plays a significant role in controlling the rate of TSR in petroleum reservoirs. A novel reaction mechanism for TSR has been proposed that involves the formation of contact ion-pairs between Mg and SO4. The formation of [MgSO4] contact ion pairs changes the symmetry of the sulfate ion thereby making sulfate easier to be reduced. This might explain the common association of H2S-rich hydrocarbon deposits with dolomitic rocks (CaMgCO3); however, it is not known whether dolomite serves as a source of Mg or rather forms as a product of TSR. Furthermore, detailed experimental work has confirmed previous reports that the presence of H2S is capable of catalyzing sulfate reduction, which significantly increases the rate of reaction. These results imply that TSR may proceed as a two-stage reaction, initially involving the slow reduction of MgSO4 until a threshold concentration of H2S is achieved, allowing for a more rapid sulfate reduction reaction that is catalyzed by H2S. While the details of the mechanisms of the H2S catalyzed reaction are still under investigation, preliminary experimental evidence indicates that the reactive sulfur content of the oil plays a significant role in controlling the rate of reaction. The recognition that the rate of TSR is strongly affected by the type of hydrocarbon involved and the concentration of the reactive sulfate may help explain why previous estimates of TSR activation energies were so divergent. This new conceptual understanding of the process of TSR in geologic environments differs from the widely held view that the initial presence of reduced sulfur (e.g., H2S or S°) is required to initiate TSR. These findings can significantly improve model predictions of H2S risk, and highlight the key controlling factors for TSR (e.g., thermal history, hydrocarbon type, reactive sulfate concentration, etc.).
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California