A Quantitative and Predictive Reservoir-Souring Approach to Assess Reservoir-Souring Risk During Waterflood Development

Souring potentials of fields during planned-/ongoing-waterflood development need to be investigated to enable the selection of the injection-water source and facility-design options. This paper presents the application of a novel reservoir-souring approach to assess the souring potential of two Middle-East fields (S and T), to recommend ways to prevent and/or reduce H2S production, and to determine the optimum solution for injection water. The novel approach includes fluid sampling and analysis, a desktop study, a dynamic-reservoir simulation, and a surface-facility evaluation. In the desktop study, a qualitative assessment of souring associated with injection-water sources (produced water and/or aquifer water) and reservoir characteristics and mitigation strategies to limit future H2S concentrations were carried out. Subsequently, a compositional non-isothermal dynamic model that includes 3 phases, 18 components, and 18 reactions was developed to quantitatively predict the most-likely and the worst-case H2S levels over the fields’ life. Several sensitivity runs were performed to assess the impact of the key uncertain parameters on the H2S level. The desktop study concluded that the produced H2S from field S has a non-microbial external source, which is likely to be derived from thermal cracking of organosulfur compounds at depth and migrated into the reservoir from the Huqf source rocks. This thermally-generated H2S is presented with an initial background H2S level in the formation water in the simulations. The Base-Case-Scenario results reveal that in the S field with the background H2S level (350 ppmv), the level of H2S increases to 1000 ppmv after injection-water breakthrough because of the addition water-induced microbial souring. In the T field without background H2S levels, the level of microbial H2S reaches 195 ppmv in year 2044 at a water cut of 95%. The results of the Worst-Case Scenarios indicate that if the VFA content is significantly underestimated and the abstraction capacity is overestimated in the Base-Case Scenarios, the risk of microbial souring would be high in the S and T fields when injecting low-salinity Fars-aquifer water. In the Worst-Case Scenarios, the gas-phase H2S concentration attains max values of 3,400 and 1,200 ppmv, respectively, for the S and T fields. Analysis of the microbial-souring mitigation options suggest that injecting the high-salinity produced-water re-injection (PWRI) at the station—being the most robust microbial-souring-prevention method available—is the best mitigation option in the T and S fields and its effectivity and efficiency are far superior to nitrate injection. In the Worst-Case Scenario, PWRI effectively hampers the generation and production of microbial H2S and maintains the H2S concentration in the produced gas around the background H2S level. Although PWRI is not an option for the S and T fields and there is no infrastructure in place for transferring the station-PWRI to the S and T fields, further analysis might justify this change of plans.