Reservoir Souring Simulation and Modelling Study for Field with Long History of Water Injection: History Matching, Prediction

Abstract

Abstract The field under study is a mature brownfield with no H2S in the fluid stream (PVT) at the time of development. However, concentrations more than 1000 ppm were recorded recently causing wells to shut in (a few already shut). Hence, to bring these wells on stream an assessment of souring potential in the field is required. This paper is the results of our experience in H2S mapping at reservoir-well-facilities modelling, history matching, and prediction of H2S. We will highlight the workflow adopted to find the root causes of souring via sampling and modelling approach since the H2S is measured throughout the field across all the reservoirs, including those undergoing waterflood. Moreover, various options that were studied through simulation will be discussed for mitigation and management of H2S within this field to safeguard the production, and thus recovery of the field. A systematic phased approach is adopted to mitigate and manage the unwanted sour gas (H2S). In the first phase, we performed the analysis of the historical development of H2S throughout the field and developed the concept for possible souring causes. In the second phase, we designed and conducted a comprehensive sampling and laboratory analysis program end-to-end to fill the existing knowledge gap. In the third phase, we performed 3D dynamic reservoir souring modelling where we history matched the H2S and assessed the future potential via forecasting. Finally, we developed multiple mitigation scenarios ranging from nitrate injection, sulphate reducing unit, limiting the nutrient supply for microbe growth via water mixing, etc. It was evident that a) increased injection water contributed to souring wells, b) link between souring wells and nutrient availability, c) increased negative fractioning of a Sulphur isotope as H2S concentration increases, and d) and mesophilic SRBs detected in some souring wells. This evidence suggested that BSR is the predominant cause of souring. It was also seen based on water chemistry that injection water was rich in sulfate while formation water was rich in volatile fatty acids. Results indicate that the nitrate injection (up to 200 ppm) alone may not be an attractive option to mitigate the H2S within this field. However, the combination of SRU and nitrate injection of 150 ppm could be a technically feasible option to mitigate such high concentration of H2S within allowable facilities limits.