Produced Water Treating Systems -Comparison between North Sea and Deepwater Gulf of Mexico

Abstract It has often been noted that water treating systems in the North Sea differ from those in the deepwater Gulf of Mexico (GoM). The objective of this paper is to provide an understanding of the reasons for these differences. In terms of platform technologies, and extraction strategies, there are fundamental differences between the two regions. FPSOs (Floating Production Storage Offloading) and fixed leg platforms are commonly used in the North Sea, whereas in the deepwater Gulf of Mexico there are currently no FPSOs, and only one fixed leg platform (Bullwinkle). Floating Spars and Tension Leg Platforms are typically used in the deepwater Gulf of Mexico. Thus, there is much greater deck space and weight availability for North Sea platforms than in the deepwater Gulf of Mexico. In addition, almost all North Sea fields are developed using pressure maintenance which relies on water and/or gas injection. Hence the water production in many of these fields has reached high water cut. In the Gulf of Mexico, there is only a handful of fields with water injection and most production is relatively dry. The approach in this paper is to compare the systems to Best Practices in water treating system design and to consider the reasons for deviation from the Best Practices. Factors which account for this deviation include capital and operating costs, extraction techniques, reservoir characteristics, the properties of the fluids being treated, the target specifications, and the obvious differences in platform type (fixed structure in shallow water versus floating structure in deep water). In addition, modeling tools are used to answer "what if’ questions. This provides a detailed understanding of the relative importance of various factors which differentiate the systems in the two regions (such as inlet fluid shear and temperature, separator flux rate, residence time, application of hydrocyclones, etc). While the overall conclusions of the analyses can be readily anticipated (i.e. deepwater systems are designed to minimize weight and space), the detailed understanding provided here gives insight into the design of water treating systems in general. It emphasizes the importance of carrying out effective water treating early in the process (i.e. in primary separation as done in the North Sea), and the necessity of using large end-of-pipe equipment when this is not possible (as in the deepwater, due to high cost of weight and space).