Abstract

This paper describes the results obtained, the techniques used, and the challenges involved in providing a water shutoff solution to a subsea gas production well with a sand screen completion on the Rose field in the UK Southern North Sea.

The project began with a detailed reservoir analysis to determine if intervention was practical in this subhydrostatic well. This led to the planning and sourcing of suitable coiled tubing (CT) and pumping equipment, as well as an associated zonal isolation chemical solution.

The design of the intervention was tailored to operate within the environment of a jackup installation located over a subsea well. This included CT and fluid pumping operations. The basis of the technique was to shut off the water-producing interval in the horizontal completion and isolate the unsupported annulus between the water bearing and gas producing intervals. The greatest challenge was to isolate the horizontal annulus between the gas and water producing zones. This required setting of mechanical barriers and the placement of suitable zonal isolation material that would set quickly without slumping and leaving the top of the annulus unsealed. This paper details the lessons learned throughout the intervention.

In the case of the Rose well, it now produces dry gas. This was the first time globally that this technique had been applied to a subsea horizontal well.

Introduction

The Rose gas field is located within the southern North Sea in UK Block 47/15b. It lies approximately 54 km northeast of Humberside in a water depth of 24m. Initially it was planned that the Rose well would be a high-rate multi-lateral well draining three separate reservoir compartments. Because not all of these compartments (North) had been proved by the discovery well the well plan was modified. The final design incorporated an extended-reach horizontal well within the North block to achieve the planned production rates. The field was eventually developed as a single-well subsea tieback. The Leman Sandstone Formation in Rose is of lower Permian age.

During 2003 well 47/15b-6w was drilled and completed with screens to prevent sand production. The well was successfully cleaned up and flow-tested, and tied back to a production platform. This platform handles production not only from Rose but from a number of other wells and has a single export line. Rose initially produced at rates of 120 MMscf/day. However, within one month of production, formation water levels were to be higher than envisaged at between 10 and 30stb/MMscf and began to exceed the capacity of the offshore and land-based production facilities. Because the offshore production platform had no testing equipment on board it was not possible to test the individual wells. Well 6w was subsequently shut in so that further analysis could be carried out. The results of this analysis confirmed that the water production was coming from the Rose well and a continuous increase in salinity was reported after production from 6w was recommenced.

The present reservoir conditions for well 6w are approximately 3,800 psi pressure, 200oF temperature, with reservoir depths of typically 9,270 ft to 9,290 ft true vertical depth.

The primary components of the upper completion are 51/2-in., 17-lb/ft L80 and 41/2-in., 12.6 lb/ft tubings and permanent production packers. The lower reservoir completion consists of two sections of 4 ½-in sand screens separated by a blank section between the upper heel, and lower toe section of the well. This is illustrated in Fig.1.

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