The Application of Holistic Workflows for Sour Reservoir Management on the X Field

The X field has been in production since 2011 and some of the reservoirs are on a water flooding-scheme for pressure support. The water flood commenced solely with the injection of seawater and more recently commingled produced water and seawater. A few years into the production, it was observed that production from at least one of the reservoirs has become sour. Thus, a reservoir souring study of the X field was conducted to ascertain the immediate cause of souring, determine future sulphide production trend and provide recommendations on sulphide concentration and integrity monitoring, bacteria monitoring practices and mitigation against the current souring events on the X field. The study was conducted by a joint team from Oriental Energy Resources Limited (OERL) - operator, Baker Hughes, a GE company (BHGE) in collaboration with the regulatory agency, the Department of Petroleum Resources (DPR). The study utilized BHGE proprietary workflows, including those previously documented in SPE-179921-MS, and drew on significant routine operational data, and specialized data such as sulphide and sulphate isotopes, detailed water compositions (including Volatile Fatty Acids -VFA), serial dilution and molecular microbiological enumeration techniques and microbiological sequencing techniques. The study concluded that the primary source of souring was microbiological and that this souring event was not limited to the focus, high H2S reservoir (XY) but was observed to be present on all water injected reservoirs in the X field. It was felt, though not proven, that the Sulphate Reducing Bacteria (SRB) may have been derived from the seawater. The microbiological Deoxyribonucleic Acid (DNA) analyses indicated that the primary carbon source, once VFA had been sufficiently depleted, was likely to be the biodegradation products of hydrocarbon degrading bacteria within the reservoir. The study also concluded that, due to the lack of carbon source constraint for the bacteria, the required inhibitory nitrate dose rates would be uneconomically high and would yield no benefit over the effects of replacing Sea Water Injection (SWI) with Produced Water Re-Injection (PWRI). However, it has been observed that sulphate concentration in production had declined since the cessation of seawater injection. The reservoir souring study has resulted in a heightened level of surveillance, thereby leading to a safer operating environment on the X asset. The field operating strategy and chemical application has been modified to allow the overall magnitude of sulphide related hazards and cost of operating the field to be reduced. This project is a first in the Nigerian oil production operations, involving an open collaboration between an operator, a government body and a service vendor on reservoir souring.