A Process Option To Comply With Zero-Harmful-Discharge Legislation in Norwegian Waters

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 118012, "The CTour Process, an Option To Comply With Zero-Harmful-Discharge Legislation in Norwegian Waters - Experience of CTour Installation on Ekofisk After Startup 4th Quarter 2007," by Kare Voldum and Eimund Garpestad, ConocoPhillips Norway, and Nils Olav Anderssen and Inge Brun Henriksen, ProPure, originally prepared for the 2008 SPE Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, UAE, 3-6 November. The paper has not been peer reviewed. A series of joint-industry projects in conjunction with Norwegian research institutions have developed a solvent extraction process to remove contaminants from produced water. Results from performance testing indicate that the efficiency from the original pilot tests had been reproduced successfully, yielding on a field scale residual oil in water (OIW) of 1 to 2 ppm and naphthalenes and polycyclic-aromatic-hydrocarbon (PAH) reduction by 86 to 92%. Introduction The Ekofisk field, operated by ConocoPhillips, is the oldest operating field in the Norwegian sector of the North Sea. It is a carbonate reservoir that has been seawater flooded since 1986. Currently, it is in a stage of increasing water cut that is projected to peak at 41 000 m3/d by year 2015. The Norwegian operators have to comply with the OSPAR (which replaced the Oslo and Paris Conventions) discharge legislation, which states that overboard discharges of hydrocarbons cannot exceed 30 ppm and that the annual total oil discharges cannot exceed 85% of the reported annual discharges of the year 2000. This represents a big challenge for an operator experiencing increasing water cut. In the Norwegian sector of the North Sea, in addition to the OSPAR legislation, operators must comply with specific Norwegian legislation that calls for "zero environmental impact" from overboard discharges. The environmental-impact factor (EIF) is used as a tool to quantify the harmful effect of discharges to the environment. The EIF was developed as a management tool on the basis of internationally agreed procedures for hazard and risk assessment. The tool models the dispersion of produced water, accounting for volume, composition of oil and aromatic components [benzene, toluene, ethyl benzene, and zylene (BTEX) and naphthalenes, PAHs, and phenols], production chemicals, and heavy metals, and it calculates the predicted-environmental-concentration/predicted-no-effect-concentration ratio. To comply with the new discharge legislation, ConocoPhillips originally projected to achieve a 95% reduction in EIF at Ekofisk by introducing produced-water reinjection (PWRI). However, after a pilot PWRI test was performed, the conclusion was that reinjection was not cost efficient, and most importantly, it became apparent that it introduced a risk of reservoir souring, and could cause substantial loss of oil produced.