Numerical Study on Tackling Microbial Reservoir Souring During Engineered Water Injection
Authors: Fathy Ahmed , Hassan Anas M. , AlAbdullah Mohammad B. , Al-Shalabi Emad W. , Rego Fabio Bordeaux , Delshad Mojdeh , Sepehrnoori Kamy 
Published: April 2024

Sulfate-reducing bacteria (SRB) activity in reservoirs causes several challenges related to reservoir souring during waterflooding. Sulfate removal units are utilized as a souring treatment solution; however, these units are expensive, and the discussion becomes more relevant when using engineering water injection (EWI) and its related benefits. In the present study, a biochemical numerical model was developed to capture a laboratory continuous up-flow packed-bed bioreactor testing using suitable microbial growth and metabolite production kinetic models. The capabilities of modeling microbial souring treatments at the laboratory and field scales during EWI were explored in this study. We employed a reservoir simulator model with a fairly simple but metabolically accurate description of competing bacterial kinetic processes. The proposed model captured the detailed mechanistic examinations of SRB and NR-SOB activities in a laboratory bioreactor alongside predicting the impacts of different influential parameters on SRB growth at a field scale model. In the absence of detailed data, the findings appear to be compatible with established characteristics of microbial growth.

The results showed that the developed 1D model was successful in history matching the increase in the generated H2S at the end of SRB growth duration in the bioreactor laboratory experiment. Moreover, the treatment was deemed successful since nitrate-reducing sulfide oxidizing bacteria (NR-SOB) commenced to grow as the nitrate was injected gradually. This resulted in complete mitigation of the H2S generated supported by the NR-SOB oxidation equation. The 1D model was tuned by division factor and reaction rate constant to match better the experimental data for H2S and H2SO4 concentrations’ change. For the 3D field-scale model, the findings showed that temperature reduction from mixing between injected and formation waters triggered H2S generation reaction and accumulation at the injector. Subsequently, it was observed that the front was moving till a breakthrough at the injector after almost 5 months where it stabilized for three months and then sharply dropped as most of the volatile fatty acid (VFA) was consumed limiting the further generation of H2S. Furthermore, SRB in the developed reservoir model seems to be more active at an optimum injected water temperature of 40 °C. Moreover, when engineered water was injected in a heterogeneous system, the generated H2S and souring onset were spiked by 2 times as opposed to that of the homogeneous system, attributed to better mixing of the engineered water injected and the formation water. This study accounts for SRB generation as well as heterogeneity and injected water temperature implications on H2S generated by engineered water injection within a unified biogeochemical model. This approach offers a straightforward yet comprehensive workflow for predicting and managing reservoir souring. By addressing fundamental mechanisms often overlooked, the proposed method brings a practical advancement to field operations and broadens the understanding of reservoir management and engineered water injection methods.