Secondary Reservoir Souring Studies and Mitigation in a Waterflood Deepwater Development – The Bonga Example – Solomon Inikori, PhD, Team Leader – Nigeria Deepwater Support, Shell Project and Technology, Houston, TX.
Bonga oils are largely defined as “sweet crude” due to absence of primary H2S. However, the corrosivity of co-produced water from an oil reservoir can change over its lifetime. This is particularly marked in fields that are initially considered sweet, but produce more H2S in later life, in some cases at concentrations of up to many thousands of parts per million by volume in the gas phase. This is quite prevalent with waterflood reservoirs where the injection of sea water into the reservoir for pressure maintenance introduces foreign elements that serve as catalysts for bacterial activities.
The Bonga Field, one of Shell’s world-class deepwater oil and gas field offshore Nigeria, was designed to produce over 200,000 barrels of oil per day with sea water injection for pressure maintenance. Routine water analysis from local laboratories at project final investment decision (FID) in 1999 did not uncover the potential for secondary reservoir souring. Nigeria government regulations prevented export of water sample for “advanced analysis” at FID hampering effective evaluation of degree of potential secondary souring. Thus, the initial Basis of Design (BoD) at FID made provision for potential corrosion of completion accessories due to co-produced water up to 500 ppm.
Advanced water analysis of exported samples in 2003 indicated high concentration of Volatile Fatty Acids (VFA) considered as substrates for the Sulphate Reducing Bacteria (SRB). Further evaluation by the Shell Water to Value (W2V) Team predicted potential secondary H2S up to 100 ppm or greater compared to 50 ppm assumed in the Basis of Design document. This new result required that a Management of Change (MoC) document be developed to introduce Nitrate Injection into the reservoir to mitigate the growth of the SRBs. The injection of nitrate introduces Nitrate Reducing Bacteria (NRB) that competes with SRBs for the same substrate thus impeding activities of SRBs. Bonga experienced water breakthrough some 2 years after first oil and has produced almost 100 MMbbls of water to date. Recent water sample analysis indicates absence of H2S.
The presentation gives an overview of the process leading to this major decision involving several hundreds of millions of dollars investments.
Speaker Bio (Solomon Inikori, PhD Petroleum Engineering Louisiana State University, Baton Rouge):
Solomon started his career with Shell Petroleum Development Company, Nigeria as a Contract Wellsite Drilling and Completions Engineer in 1991. He moved to a Nigerian Independent Oil and Gas Company (Allied Energy Resources, Nigeria, in Alliance with British Petroleum and Statoil) from 1995 to 1998 where he worked on both deepwater and shallow offshore drilling and completions projects including a 3-month training on deepwater development in the North Sea, Norway. He rose to the position of Engineering Coordinator leading the development of shallow offshore Obe field to first oil.
Between 1998 and 2002 Solomon was a graduate student at the Louisiana State University, Baton Rouge and a Consultant Drilling/Reservoir Engineer with Allied Energy Corporation, Houston. He is the recipient of SPE First Place prize of 2002 PhD student paper contests at both the Gulf Coast Region and International level and LSU College of Engineering exemplary dissertation award.
He joined Shell International E & P, Houston in July, 2002 as a Senior Reservoir Engineer. He has supported the development of several deepwater fields through including Brazil’s BC-10; Nigeria’s Bonga Main, Bonga IFOs, Bonga NW, Bonga SW and Bonga North. He also worked on the Erha Main and Erha North fields operated by ExxonMobil with Shell as a non-operator (43.75%) leading to Erha North Phase 3 FID in 2012. He led the subsurface team in the evaluation of Shell Canada’s Sable Island Significant Discovery Licenses (SDL) leading to divestment in 2009. He is currently Leader of the Nigeria Deepwater Support Team working the development of Bonga SW and Bosi Integrated Oil and Gas Fields – over US$30bln oil and gas developments in planning.