Subsea Oil/Water Separation: Main Drives and Challenges

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 97375, "Application of Subsea Oil/Water Separation: Main Drives and Challenges," by M.W. de Figueiredo, Petrobras, prepared for the 2005 SPE Latin American and Caribbean Petroleum Engineering Conference, Rio de Janeiro, 20-23 June. Petrobras is developing a subsea system for oil/water separation to minimize the effect that high water-production rates cause to the topside facilities when waterflooding an oil field. Most of the produced water can be separated at the seafloor and reinjected into the reservoir or into a water-bearing formation. Introduction During the exploitation-design phase of an oil field, activities focus on oil-production optimization. The oil-recovery mechanism is defined to maximize the recovery factor according to the reservoir and fluid characteristics. In most of the oil fields in Campos basin, waterflooding is used from the beginning of production. After water breakthrough occurs, the handling of produced water, which is a mixture of injected seawater and reservoir water, is of secondary importance during the design phase. Its priority increases only after water production increases. Conventionally, this water is separated from the oil and disposed of into the sea. This procedure takes place onboard the floating production units (FPUs) by use of very-large-volume vessels on a deck where space is critical, especially with the increasing plant capacity of recent units. The use of a subsea system to separate most of the produced water at the seabed and send it somewhere other than the FPU plant is an attractive option. Such a system would allow the production plant to continue handling only a residual amount of water, eliminating new construction onboard to increase water-handling capacity. This system could enable adding extra oil-processing capability and, in many cases, increase oil production because of the lower wellhead backpressure that results from reduced water-lifting energy requirements. Subsea separation presents challenging issues such as compactness and reliability. The full-length paper details this development, including the most promising scenario and the main benefits and challenges related to this scenario.