Use of Nitrate to Mitigate Reservoir Souring in Bonga Deepwater Development Offshore Nigeria

Abstract

The Bonga field located in deep water off the Nigerian coast needs pressure support to effectively recover the hydrocarbons. The strategy is to inject 300k bwpd 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 expected H2S content resulting from reservoir souring was not expected to exceed 50 ppm(v) in the gas phase. Initially nano-filtration to reduce the sulphate level in the seawater was identified to mitigate reservoir souring but due to the high CAPEX costs it was dropped and as 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 H2S with sour service materials and scavenging facilities topside. The facilities were designed to handle a maximum level of 50 ppm(v) H2S. 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 H2S at surface was re-evaluated in 2003. It was found that H2S 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 H2S development. However, to date the main application for nitrate had been the reduction of H2S 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 due to biogenic reservoir souring in the Bonga field, and work done to predict H2S levels. The paper will also focus on the selection of nitrate as a mitigation method. Introduction The Bonga field (Figure 1) lies on the continental slope in the southern part of the Niger Delta some 120 km offshore, South West of Warri in Nigeria with water depths ranging from 950 m to 1500 m. The reservoirs are Lower Upper Miocene in age, and are interpreted as stratigraphically / structurally trapped mud rich unconfined turbidite systems in a mid-lower slope setting. The reservoirs are composed of fine-grained amalgamated channel sands derived from the shelf margin to the northeast. The main 702 reservoir, which is expected to deliver over half of the recoverable reserves, is comprised of amalgamated turbidite channels. The other reservoirs are stacked either above (690) or below (710/740, 803), and are generally less well amalgamated. Net reservoir thickness is generally less than 100 ft. Measured sand porosities range from 20–37% and are generally associated with high (multi-Darcy) permeabilities. Seawater injection for pressure maintenance and sweep is key to the success of the Bonga development. A total of sixteen wells (nine oil producers and seven water injectors) were drilled during the Bonga Phase 1 drilling campaign. All fluids produced will be processed on an FPSO situated centrally in the field and oil is directly loaded to tankers (Figure 2). The associated gas will be exported through pipelines. Water will be processed to appropriate standards and disposed of overboard.

Acetate Production from Oil under Sulfate-Reducing Conditions in Bioreactors Injected with Sulfate and Nitrate

“…Pham et al (55) found up to 3.6 mM acetate in produced waters from a mesothermic Alaskan reservoir, and Kuijvenhoven et al (29) reported up to 15 mM acetate in waters produced from seawater-injected oil fields off the coast of Nigeria and in the North Sea. Produced waters from the high-temperature Kaparuk field had acetate concentrations of 8.3 to 14.7 mM (56).…”

“…Both NRB activities produce nitrite as an intermediate, inhibiting dissimilatory sulfite reductase Dsr, the highly conserved enzyme that produces the sulfide (22)(23)(24). In applying this biotechnology (25)(26)(27)(28), volatile fatty acids (VFA; e.g., acetate, propionate, and butyrate) are often considered to be the predominant electron donors for nitrate reduction and decisions with respect to the nitrate dose to be used take VFA concentrations in produced waters into account (29). Direct use of oil components, especially toluene, as electron donors for nitrate reduction has also been demonstrated (30,31,32).…”

Water Compatibility Study: Detecting Souring Potential

“…In the North Sea, H2S concentration of 1.5tonnes per day with a maximum wellhead concentration of 1,000ppmv has been recorded while 40,000ppmv has been reported in Huntington Beach Field in California [8]. Reservoir souring has also been reported in the Bonga field of the Gulf of Guinea where reservoir temperatures are around 63 o C and TDS is well below the maximum limit of 15 to 20% [9]. Methods of militating against reservoir souring have been discussed [10,11].…”