The Bonga field, located in deep water off the Nigerian coast, needs pressure support to effectively recover hydrocarbons. The strategy is to inject 300,000 BWPD of seawater from the start of oil production. During the field development in 1999, it was concluded that Bonga was expected to suffer from reservoir souring and that mitigation would be necessary. Initial data gathering indicated that the H_2S content resulting from reservoir souring was not expected to exceed 50 parts per million(volume-based) ppm(v) in the gas phase. Initially nano-filtration to reduce the sulfate level in the seawater was identified to mitigate reservoir souring, but because of the high capital-expenditure (CAPEX) costs, it was dropped and, because there were no other proven mitigation techniques available, it was decided to operate without mitigation. The strategy for this project was to let the reservoir sour and handle the H_2S with sour-service materials and scavenging facilities topside. The facilities were designed to handle a maximum level of 50-ppm(v) H_2S. As detailed design progressed and more field data became available, doubts were raised on the suitability of this approach. The strategy to let the reservoir sour and handle the H_2S at surface was re-evaluated in 2003. It was found that H_2S levels are likely to exceed 50 ppm(v). Since then, a new strategy with mitigation was adopted. Several operators had verified that nitrate injection is an effective mitigation technique to control H_2S development. However, to date, the main application for nitrate had been the reduction of H_2S in already-sour fields, and the experience for the use of nitrate from the start of the water-injection scheme was limited. This paper presents a detailed evaluation of the potential for reservoir souring resulting from biogenic reservoir souring in the Bonga field and the work done to predict H_2S levels. The paper focuses on the selection of nitrate as a mitigation method.
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