Abstract

Gel is considered to have an interesting potential for water shut off applications, especially in fields having natural shale barriers and large permeability variations and/or where reduction in tubing/casing diameter is not wanted. This paper describes a case study from the Statfjord Field where polymer silicate gel was used to build up experience for water shut off applications of the lower producing interval in a well that produced from two intervals. The intervals where separated by 5m in 9 5/8" casing and located at approximately 3100 m.

The primary objective was to shut off water and secondly to evaluate gel technology. Mechanical techniques and gel technology were considered as means of achieving the objectives. Both required the use of 2" coiled tubing in a live well (Fig. 14). Gel technology was selected since diameter restriction in the casing was not wanted due to potential later perforations deeper into the well combined with desire of qualifying the technology and related techniques.

This paper reports some of the results achieved in the laboratory, and give references to papers where the laboratory techniques and results are given, refs. 1 through 5. The pre planning and the laboratory testing for finding a suitable gel formulation was also performed in collaboration with a service company.

Therefore this paper focuses mostly on the well performance, placement of plugs in the well by wireline and coiled tubing, pumping of the gel solution through the coiled tubing and into the lower perforation interval and evaluation of results. The operation has been an economical success. After water isolation from the lower interval, the well has produced oil with a gross value of 15 mill. U.S.$ in 6 months and the operation had a pay back time of 50 days.

Introduction

The Statfjord Field is one of the largest offshore fields in the world. It is located on the boundary line between the Norwegian and British continental shelves, 200 km North West of Bergen. Production started in 1979 from one concrete gravity based production platform. Field development was completed in 1985 with three fully integrated platforms based on Condeep design. Each platform has two drilling shafts and a total of 42 well slots. To date 121 wells have been drilled including 19 redrills Maximum production of 120000 Sm3/d was reached in 1987 Production has been declining since 1993 and is presently around 78000 Sm3/d. Approximately 80% of recoverable reserves have been produced. These are estimated to be 620×106 Sm3 of oil. The field covers an area 25 km long and 4 km wide. Production comes from Jurassic sandstones. The main reservoirs are Upper Brent (Tarbert and Ness), Lower Brent (Etive, Rannoch and Broom) and the Statfjord formation (Fig. 1). Production from Brent wells is mainly limited by sand and some wells are also affected by high water cuts.

The well 33/12-B39 was producing with a watercut of approximately 70 % at the end of March 1995.

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