Potential Risk Evaluation of Biogenic Reservoir Souring for Decision Support of Water Flooding Field Development Plan in a Giant Offshore Oil Field in Abu Dhabi

Abstract

Abstract Biogenic reservoir souring is characterized by H2S generation by Sulfate Reducing Bacteria (SRB) typically after the initiation of waterflooding. An offshore oil field in UAE has been developed with dump flooding from an aquifer reservoir and peripheral seawater injection. As the future development plan (FDP), pattern seawater injection in the mid dip was selected to improve oil recovery. To assess the robustness of this development plan, the study aims to understand the mechanism of biogenic reservoir souring and predict its future risk. Firstly, analytical investigation was conducted to assess the present reservoir souring situation. Secondly, the full-field biogenic reservoir souring simulation was performed using SourSimRL (industrial JIP's program) that works as a post-processor of reservoir flow simulators and predicts the microbial generation, scavenging, adsorption and transport processes of H2S. Lastly, sensitivity analyses were carried out to understand the limiting factors of the biogenic reservoir souring and capture the uncertainties in the prediction of the FDP. The simulation demonstrated that the reservoir cool-down effect by injecting seawater is limited to the vicinity of injectors due to the high reservoir temperature. Thus, dominant pattern of biogenic reservoir souring is expected to be "biofilm" zone where all ingredients for SRB growth need to be supplied with the injected seawater. The sensitivity analysis also confirmed that carbon source and nutrient concentrations in the injected seawater have a larger influence on biogenic H2S generation compared to those in the formation water. The forecast simulation predicted the gradual increasing trend of biogenic H2S concentration in the produced gas at the field level, which is corresponding to the field water cut increase. However, the severity of predicted biogenic reservoir souring impact is still within the facility design limitation. Thus, the FDP is acceptable in terms of biogenic reservoir souring risk because of the robust design basis. Additional souring mitigations such as installation of sulphate removal unit is considered unnecessary. Although the field-level biogenic H2S is moderate, relatively high biogenic H2S concentrations were predicted locally in several wells. It can be managed by well surveillance at the high water cut wells. The study managed the complicating field development history, consisting of dump flooding from the aquifer reservoir and peripheral seawater injection, and the FDP deploying pattern seawater injection in the mid-dip. Application of the full-field biogenic reservoir souring simulation significantly contributes to decision support of the water flood-basis FDP and better understanding on potential requirement to mitigate the reservoir souring risk.