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

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 159713, ’Produced-Water-Treating Systems: Comparison Between North Sea and Deepwater Gulf of Mexico’ by J.M. Walsh, SPE, Shell, and W.J. Georgie, SPE, Maxoil Solutions, prepared for the 2012 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, 8-10 October. The paper has not been peer reviewed. In terms of platform technologies and extraction strategies, there are fundamental differences between the North Sea and the deepwater Gulf of Mexico (GOM). We consider these systems in light of best practices in water-treating-system design and consider the reasons for deviation from those practices. This process provides insight into the design of water-treating systems in general, emphasizing the importance of carrying out effective water treatment early in the process and the necessity of using large end-of-pipe equipment when this is not possible. Introduction Generally speaking, there is a perception that water-treating systems in the North Sea are different from those in the deepwater GOM. Specifically, North Sea systems involve greater use of hydrocyclones, with either no flotation or compact vertical flotation. Deepwater GOM systems are associated with the use of hydrocyclones (but to a lesser degree than in the North Sea) and the use of large horizontal multistage flotation. While these differences are often noted, even greater differences are observed by considering the oil/water/gas-separation system as a whole. The oil/water-separation system includes dehydration of oil and deoiling of water. From the standpoint of the overall integrated oil/water-separation system, the water-treating system is only a part. If a water-treating system is considered an add-on to the overall separation-system (oil/water/gas) design, rather than an integral part, then it will invariably be tasked with a much more difficult challenge (smaller droplets, more-stable emulsions). In that case, it will require large multistage flotation and perhaps tertiary end-of-pipe equipment such as media treatment or filtration. As shown in Fig. 1, a typical North Sea oil/water-treating system consists of inlet heating where necessary (not shown), followed by first-stage three-phase separation. The produced water is separated from the first-stage separation with enough pressure and is treated through hydrocyclone units. As shown in Fig. 2, typical deep-water GOM oil/water systems consist of one- or two-stage two-phase separation, followed by three-phase separation. Inlet heating is often not used. The produced water is separated toward the end of the process at lower operating pressure. The produced water is processed through hydrocyclones through a common collecting vessel operated at a lower pressure and, in many cases, a lower temperature.