Effects of gas saturation and reservoir heterogeneity on thermochemical sulfate reduction reaction in a dolomite reservoir, Puguang gas field, China

Abstract

Varying concentrations of H2S have been found in the carbonate reservoir, Puguang gas field, China. Thermochemical sulfate reduction (TSR) is considered the most important process to generate a high level of hydrogen sulfide (H2S) in the reservoir. Although TSR has been intensively studied in this field, it is still not clear on (1) the controls of H2S concentration variations in the reservoir, (2) effects of variable gas and water saturations and reservoir heterogeneity on the TSR reactions, (3) reservoir quality changes during the TSR reactions, (4) potential sources and sinks of sulfur and carbon and their quantification. In this study, we conducted reactive transport modeling to quantitatively simulate the TSR and H2S generation processes in the Upper Permian Changxing Formation and Lower Triassic Feixianguan Formation in Puguang gas field, using log-based depth and thickness, paleo mineralogy assemblages and paleo temperature in late Jurassic, and hydrostatic pressure. The modeling results are relatively reliable as the predicted H2S and CO2 concentrations are comparable with the observed ones in the wells after calibration. The results indicated that both mineralogical composition and gas saturation in the reservoir affect the local efficiency and extent of TSR reactions, of which the abundance of effective anhydrite is an important control on the extent of the TSR reactions. The modeling durations of TSR reactions were ∼6 m.y. and TSR had a small impact on the reservoir porosity (porosity changes generally less than 0.1 porosity unit). The sources and sinks of carbon and sulfur were locally balanced with each other, respectively, and the minor imbalances were likely due to the molecular diffusion. The calibrated models have predictive potential on the H2S and CO2 concentrations changes and porosity modifications during the TSR processes in the subintervals with limited samples. The validated TSR models will facilitate the investigation of organic-inorganic interactions and their impacts on both non-hydrocarbon (H2S and CO2) generation and reservoir qualities.