ABSTRACT

The seawater nanofiltration process removes the sulfate component in injected seawater for oilfield waterflood operations. Benefits of the sulfate removal process include oil flow assurance by the elimination of scaling and well souring caused by the conversion of the sulfate to hydrogen sulfide by thermophilic sulfate reducing bacteria. Since its initial use on the Brae Alpha Platform in the North Sea for scale control, the nanofiltration process has found applications in other oilfield applications, has had technology improvements that have reduced its cost, and has provided exceptional benefits in complex reservoir developments.

INTRODUCTION

Early in the South Brae oil reservoir development, Marathon Oil realized that pressure maintenance with seawater would be required over the reservoir life. It was also discovered that the Brae reservoirs contained between 800 and 2,500 rag/1 barium in the formation water. By injecting seawater containing approximately 2,700 mg/1 sulfate into these reservoirs, severe scaling potential was anticipated due to the reaction of the sulfate in the seawater with the barium in the formation water. This was further complicated by the presence of radioactive radium 226 and 228 in the Brae formation water. Since the radium would also be precipitated by the sulfate in the injected seawater, a combination of barium and radioactive radium sulfate would be formed. This combination would (1~ Consultant to Pratt Technology Management, Formerly Manager of Commercial Technology for Marathon Oil. Littleton, CO 80122 result in a NORM (Naturally Occurring Radioactive Material) scale which presented handling, safety, and disposal problems. In addition, production tubulars completely blocked with scale were removed and taken to shore where the NORM scale could be milled from the tubing. Needless to say, this resulted in expensive workovers.

With the high levels of barium in the Brae formation water, existing scale inhibitors simply were stretched beyond their capabilities and unable to prevent the precipitation of barium sulphate and resultant scaling. Further complicating factors associated with the Brae reservoir in the use of existing scale inhibitors, were the low pH of the formation water and high calcium levels 1. In an effort overcome these difficulties with the existing scale inhibitors, a series of sulphonate-based inhibitors, which would better meet the demanding requirements of the Brae reservoir, were developed.

In addition to the improved sulphonate based inhibitors that worked well at reduced sulfate concentrations, Brae operations additionally took an innovative approach by examining the possibility of removing the sulfate from the injected seawater in order to render the sulfate scale problem manageable 2. Hence, the source of the scaling was dramatically reduced versus the traditional approach of attempting to control its precipitation in the supersaturated barium sulfate formation water entirely with scale inhibitors.

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