Reservoir souring: sulfur chemistry in reservoir

Abstract

Reservoir souring is a widespread phenomenon in reservoirs undergoing seawater injection. During this process, the sulfate in the seawater promotes the growth of sulfate reducing bacteria and archaea generating hydrogen sulfide. However, other reactions involving formation of different sulfur species with intermediate valence states such as elemental sulfur, sulfite, polysulfide ions, and polythionates can occur. These sulfur species have implications in both chemical and microbial processes and impact the treatment approaches for soured reservoirs. A predictive reactive model was developed in this study to investigate the chemical reactivity of sulfur species and their partitioning behaviour as a function of temperature, pressure, and pH in a seawater-flooded reservoir. The presence of sulfur species with different oxidation states impacts the amount and partitioning behaviour of H₂S. The model predicts at pH values less than 5, sulfur is predominantly in the form of sulfate, polysulfide, hydrogen sulfide and approximately 10% of the total sulfur is thiosalts. There is also elemental sulfur precipitation. At pH above neutral, the bulk of the sulfur is sulfate and thiosalts and less than 10% polysulfide is formed. The amount of sulfur deposited as elemental sulfur is also less compared to pH lower than 5. Without considering sulfur speciation and assuming that all the initial sulfate in the injected seawater (2454 mg/L) is converted to H₂S microbially and the formed H₂S partitions between phases, the gas phase under test separator conditions on the surface contains 1080 ppm H₂S which is in equilibrium with the oil phase containing 295.7 ppm H₂S and water phase with H₂S content of 8.8 ppm. These values are higher than those obtained from reactivity analysis (i.e., H₂S content of the gas, oil, and aqueous phases are 487 ppm, 134 ppm, and 4 ppm, respectively). Therefore, ignoring sulfur speciation in investigating reservoir souring leads to over-prediction of H₂S content of reservoir fluid. The developed reactivity model enables one the investigation of sulfur chemistry when injection of sulfate-containing seawater is used in recovery processes. This model can be used as a tool to study sulfur speciation and H₂S amounts as a function of reservoir temperature, pressure and pH and therefore, could be used in developing souring mitigation measures.