Dynamic Modeling Approaches to Forecast and Mitigate Maximum H2s in Sour Crude Oil Reservoirs: A Case Study from Waterflood Fields in Oman

Abstract The production of sour crude oil often results in the release of hydrogen sulfide (H2S) into surface equipment, leading to compliance, safety, environmental, and corrosion issues, as well as significant costs for upgrading equipment to sour service. To mitigate H2S formation and minimize its impact on both downhole and surface equipment, it is crucial to understand its source and predict its development within reservoirs. This study utilizes dynamic reservoir simulation to forecast H2S generation and transport in waterflood fields, providing a framework for designing future wells and surface facilities. A compositional reservoir simulation model was developed, incorporating 17 components, 3 phases, and 18 reactions to simulate microbial H2S generation and transport. The model integrates environmental factors (e.g., temperature, salinity, pH) and reservoir conditions (e.g., permeability, H2S partitioning) to simulate microbial souring. Metabolic reactions, including sulfate and nitrate reduction, driven by nutrients such as BTEX (benzene, toluene, ethyl-benzene, xylene) and volatile fatty acids (VFAs), were parameterized to capture microbial activity. The model was calibrated using field data from waterflood fields in Oman, focusing on the interaction between microbial souring and reservoir dynamics. The study reveals that post-water breakthrough H2S levels are influenced by initial background H2S concentrations, nutrient availability, rock abstraction capacity, and reservoir heterogeneity. Base-case scenarios show that fields with existing background H2S maintain stable levels, while fields without background H2S experience microbial souring, with H2S concentrations increasing by the end of the simulation. Sensitivity analysis identifies VFAs as the most influential carbon source for microbial activity, with rock abstraction capacity playing a critical role in H2S scavenging. Worst-case scenarios emphasize the risk of significant microbial souring if VFA content is underestimated or abstraction capacity is overestimated, particularly when injecting low-salinity water. The findings underscore the importance of accurately measuring VFA content and rock abstraction capacity for effective souring prediction. The study recommends high salinity produced water re-injection (PWRI) as the most robust mitigation strategy, effectively suppressing microbial H2S generation and maintaining H2S levels near background concentrations. This work provides a practical framework for predicting and managing H2S souring in waterflood fields, offering valuable insights for reservoir engineers and field operators to optimize field development plan (FDP) and mitigate souring risks.