Produced Water Reinjection: Experiences From Performance Measurements on Ula in the North Sea
Authors: Hjelmas T.A. , Bakke S. , Hilde T. , Vik E.A. , Gruner H. 
Published: December 2007

Abstract
BP Norge Ltd. was the first North Sea operator that with a minimum of capital investments attempted to eliminate produced water discharges through the reinjection of mixed produced water and seawater. A full scale produced water reinjection (PWRI) trial started the first quarter of 1995. Performance measurements have included the use of models and documentation of the effects with respect to preventing loss of injectivity. corrosion. scaling, as well as reservoir souring, which have been experienced in previously reported PWRI trials. This paper includes results from the first year of reinjection. It will hopefully promote the industry to consider PWRI as a viable alternative which should be closer evaluated and to provide a foundation for optimising water handling facilities in the future.

Introduction
Increased environmental concern for the effects of produced water discharges encourages oil producers to consider improving performance at a time when profit margins are small. In addition, it is in the nature of many production systems that the costs of produced water handling increase significantly as the oil field matures when at the same time revenues are in decline.
Produced water reinjection (PWRI) is perceived as the most likely method for eliminating the environmental impact of produced water at offshore oil production sites. There is a potential for making cost, space and weight savings through optimisation of water treatment facilities and produced water reinjection system during the life of a field.
Reinjection of produced water has been carried out on several locations around the world. BP has, for instance, performed reinjection of produced water in Alaska (Prudhoe Bay) and in the UK (Forties and Wytch Farm). In most cases the activities have been concentrated on individual wells and have not included mixing the produced water with seawater prior to injection. The experiences from these trials have been variable. In most cases some loss in injectivity has been seen, in some cases the problems have been more severe, i.e. accelerated reservoir souring and increased scaling have also been noted.
The results obtained from these BP sites have stressed the need for a better understanding of all mechanisms that influence the impact of PWRI. A number of studies are underway in various parts of the world, aimed at creating a better predictive capability of the impact of PWRI in specific field cases. BP Norge (BPN) has been active in these studies and is a member of a UK based produced water reinjection joint industry initiative which is supported by six oil companies. It is an integral part of the Norwegian Research Council's MUST (Environmental and Profitable Development of Small Fields) programme to work closely with these initiatives which provides the opportunity of verifying the relevant predictive models.
BPN has accepted that PWRI may be the only way to reduce the produced water discharges, although considerable uncertainty about the costs of implementation and the consequences of reinjection still exist. BPN therefore completed a PWRI trial in one single well on the Ula field in 1994 and based on these results, continued with a full scale PWRI trial in 1995.
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