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Abstract

This paper describes the introduction of a new perforating system in Norway, including its development based on operator specifications. Field examples demonstrate the measurable success of the system, showing a three- to sixfold increase in productivity and production rates as high as 8500 m³/D per well.

The Visund field is located in the North Sea, about 150 km off the coast of mid-Norway. High reservoir pressure and insufficient formation strength made it necessary to perforate in relatively heavy completion fluid with the guns oriented for sand prevention. The first wells, perforated in water-based fluid with zinc charges, produced below expectations. Potential causes were identified as insufficient depth of penetration, high overbalance when perforating, and zinc precipitation damaging the formation. There were also indications from other fields that the orienting technology in use was not producing expected results.

The operator started an initiative to improve the perforating system. Design criteria included deep-penetrating, low-debris steel charges that minimize debris and improve performance, increased gun-void volume for a better surge effect, accurate orientation of the perforation tunnel regardless of dogleg severity, and confirmation of the orientation. The operator and the service company collaborated to develop a system that would provide a better solution. The system was tested and qualified before it was introduced in the Visund field. To optimize perforation cleanup during static overbalanced well conditions, a dynamic underbalance method was selected.

Since December 2001, six wells have been completed on Visund using the new system. In one well the dynamic underbalance response was confirmed from downhole pressure gauges run directly above the guns. The combined benefits of the perforating system, the dynamic underbalance, and a special kill pill have resulted in up to sixfold increase in productivity. The gun orientation is verifiably within requirements, and perforating debris have been minimal.

Introduction

The Visund field is a subsea development (Fig. 1) about 150 km off the coast of mid-Norway. It was put on production in April 1999 by Norsk Hydro ASA. On the 1st January 2003 the field was taken over by Statoil ASA as part of an agreement. Visund has proven to be a highly complicated reservoir to exploit, with a complex geology containing many surprises. The heterogeneous fault-segmented reservoir consists of Jurassic fluvial and deltaic sands with the main pay zone permeabilities ranging from 300 to 3,000 md. The wells, as are an increasing number of wells in the North Sea, were drilled and completed with long horizontal sections to reach several targets with one well. A typical well path is shown in Fig. 2, which illustrates the complex sinusoidal well paths required. The relatively high reservoir pressure and temperature of 440 bar and 115°C at mean sea level (MSL) of 2950 m true vertical depth further increased the drilling complexity.

The Visund wells are completed using a 7-in. monobore completion in a 10 3/4-in. casing (Fig. 3). The unconfined compressive strength of the weak formations varies between 5 and 20 MPa and requires some method of sand exclusion. Because the lower range formation strength is 5 MPa, oriented perforating in the direction of the maximum stress can be used as a sand prevention technique for these wells.^1-3 The alternative would be to complete the wells with screens for sand control. The drilling time of these complex horizontal wells is relatively long, which results in a long exposure of high overbalanced drilling fluids to the formation, which in turn produces a deep mud filtrate invasion zone around the wellbore. Therefore screens sometimes result in poor productivity. Perforated cemented liner can be used with deep-penetrating charges to bypass this damaged zone and provide communication with the virgin formation rock.