Subsea Water-Treatment System Installed in Pilot Test at Ekofisk

Abstract

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper OTC 29552, “Subsea Water Treatment Pilot Testing at the Ekofisk Field in the North Sea,” by Eirik Dirdal, National Oilwell Varco, prepared for the 2019 Offshore Technology Conference, 6–9 May, Houston. The paper has not been peer reviewed. Copyright 2019 Offshore Technology Conference. Reproduced by permission. A subsea water-treatment system designed to leverage space availability, steady temperatures, lower bacteria levels, and other natural benefits of the seabed environment was successfully installed during pilot testing at the Ekofisk field offshore Norway. After several phases of testing, a full-scale water-treatment module was built and installed, and at the time the complete paper was written, the pilot had moved into the long-term testing phase to determine the system’s viability under North Sea field conditions. According to the author, if the system succeeds, it could drive a major change in injection-water management offshore. Introduction The Ekofisk field was originally developed with pressure depletion as the primary drive mechanism. After several decades of production, significant resources are still in place. Limited gas injection and comprehensive water injection have contributed to a substantial increase in oil recovery. Large-scale water injection began in 1987, and has been extended in several phases. Water displacement of the oil has been successful, and the recoverable resources have been adjusted correspondingly upward from the original 17% to an estimated final recovery factor of greater than 50%. An important factor in the success of a waterflood project is the quality of the injected water. In worst-case scenarios, poor water quality can cause plugging and reservoir souring. Water quality is greatly affected by contaminants such as suspended solids, corrosion products, scale, and bacteria. Controlling corrosion and disinfection of biomass are two major challenges associated with waterflood. The two challenges are linked because oxidants are required to reduce bacterial activities that could lead to microbial-induced corrosion, but the oxidants also promote corrosion. Joint-Industry Project (JIP) Results Development of the injection-water treatment system described in the paper began in 2004. Det Norske Veritas’ recommended practice for qualification of new technology has been used throughout the development process. On the basis of successful experimental testing that demonstrated proof of the concept, a JIP supported by several oil and gas majors was launched in 2009. A prototype was built and tested in 65 m of water depth in Oslofjorden, a sheltered environment in a fjord, to verify the water-treatment module’s performance and reliability through seasonal variations in real offshore conditions. Results demonstrated that the treatment method, which combined various electrolysis approaches, provided very good water quality, in many aspects superior to that achieved by topside seawater-treatment plants.