Mitigating H2S at Offshore Malaysia's Mature Oilfield: Pilot Nitrate Injection Evaluation
Authors: Ismail W. , Shah T. Tengku Zainal Alam , Mawardi M. , Lajawai M. T. , Tahir M. M , Bak A. A , Koyyalamudi L. , Runham G. , Rosli M. , Azman N. , Nianamuthu M. F. 
Published: September 2024

Mature oil fields under seawater waterflood scheme are prone to souring and H2S production by reservoir souring due to H2S production by sulfate-reducing bacteria (SRB) that threatens to production, facilities, environment, and the people. This study aims to tackles this challenge ahead-on, presenting a comprehensive evaluation and optimization of a pilot nitrate injection strategy deployed in a mature Malaysian field afflicted by souring. With the introduction of nitrate injection to the field, the operator aims to reduce H2S concentrations from a high of 2000ppm to near zero. Such a feat would not only safeguard production but also unlock substantial economic benefits. By curbing H2S, the team potentially avert the escalating costs of topside H2S scavengers, and the major facilities upgrade for ten wells spewing 2000ppm of toxic gas. This translates to a direct financial reward for the field, while simultaneously protecting the environment and ensuring operational longevity.

To assess the effectiveness of the pilot nitrate injection, an integrated approach combining laboratory experiments, numerical modeling, and field monitoring was employed. Literature studies were conducted to determine the optimal nitrate concentration and injection rate for suppressing SRB activity. The obtained results were then used to design the operational aspects of nitrate injection and performance monitoring in the absence of lab studies. Field monitoring involved the collection and analysis of production fluids from both injection and production wells. The H2S concentration, total microbial population, and microbial community composition were monitored overtime using novel microbial monitoring technique of next generation sequence (NGS). These measurements provided insights into the extent of reservoir souring mitigation achieved by the nitrate injection and the long-term impacts on microbial activity.

The results of the pilot study demonstrated the effectiveness of nitrate injection in managing reservoir souring, with significant reductions in H2S concentration observed, indicating successful suppression of SRB activity. Microbial analyses revealed changes in microbial community composition, with a decrease in SRB abundance and an increase in nitrate-reducing bacteria (NRB). These results then further correlate with the changes of produced water composition pre and post injection which indicated reduction in volatile fatty acid (VFA) consumed by NRBs.

The findings from this pilot study provide valuable insights into the feasibility and optimization of nitrate injection as a reservoir souring management strategy in mature oil fields. The results can be utilized by oil field operators and engineers to design larger-scale nitrate injection programs and develop mitigation strategies to maintain reservoir productivity, enhance infrastructure integrity, and ensure environmental safety in souring-prone oil field.

This paper is a continuation to SPE-216234-MS: Reservoir Souring Management in A Mature Waterflooded Offshore Field Malaysia: Case Study of Field-D that was presented during ADIPEC 2023 in Abu Dhabi as continuation of reservoir souring management at Field-D.