Overcoming the Metering Challenge in West African Deepwater Subsea Developments

Abstract

As more subsea projects mature and with increasing understanding of operations involving subsea wells, the industry is constantly faced with the constraints and challenge posed by the requirements for metering, sampling, well testing and allocation. These challenges are even more pronounced in cases where flow assurance issues play a role especially for long tie-back systems. Current regulation in the Nigerian sector has yet to catch up with these developments and as such operators struggle to meet the requirements. A number of possible solution to the seeming problems or issues are under study, and with updated knowledge of the systems, it has become imperative to articulate and formulate strategies and philosophies for resolving or addressing the metering challenge for operations in Deepwater Nigeria. This article analyzes one of such solution in the application of subsea multiphase flow meters Introduction Subsea systems have evolved over the years, as a major solution to the technical and commercial challenge of harnessing reserves in the deepwater terrain. With the ability to extend tie-back distances, it affords a perfect means of achieving hub strategies allowing for continual development of reservoirs at great stepout distances. As stepout distances increase so does the challenge of delivering the hydrocarbon to the host facility. Flow assurance issues are key and often compounded by unwanted deposition such as hydrates, wax, scale and asphaltenes in the transportation and processing system components. This makes the task of measuring and quantifying the volumes of hydrocarbon products difficult. The traditional method is to measure these product quantities at the host, via test separators. The accuracy of the flowstream measurements is however a function of the stability of the system, requiring the introduction of artificial lifting systems (in most cases riserbase gaslift is employed). Operational experience with subsea production testing reveal that there are threshold rates required for stability. Below these threshold, serious system upsets such as slugging, impacts on the production system sometimes causing a shut down of the facility.