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Abstract

In an effort to minimize capital investment in platform installations there is an increasing trend platform installations there is an increasing trend towards drilling subsea satellite wells, with production tied back to an existing platform. A production tied back to an existing platform. A disadvantage with this type of well is that regular access for maintenance and reservoir monitoring is denied by the high cost of conventional techniques.

This paper, which discusses some new techniques for economic monitoring and maintenance of subsea wells, is divided into two sections.

The first section describes the installation and use of permanent gauges run with the completion in subsea wells. Methods of mounting the pressure gauge in the completion string and transmission of the data to surface are discussed. A field case is reviewed which highlights some of the advantages of continuous reservoir monitoring.

The second section of the paper describes a newly-developed technique for wireline logging in wells completed subsea. The method involves a specially designed lubricator and pressure-control system which can be deployed from a dynamically positioned support vessel. The discussion is focused positioned support vessel. The discussion is focused on the practical aspects of the technique, as well as giving an overview of the types of operations that can be performed.

Introduction

The number of subsea wells drilled in the U. K. sector of the North Sea has increased considerably since 1975. Figure 1 illustrates this growth by showing the cumulative number of subsea wells in existence at the end of each year since 1975. Projections for the number of subsea wells over the Projections for the number of subsea wells over the next five-year period indicate a continuing increasing trend.

Since the North Sea is a mature province, in many cases new discoveries, or previously unexploited small accumulations, can be produced to an existing platform. The economic viability of these so-called "marginal fields" largely hinges on the reduced capital expenditure which can be realised by utilizing existing facilities. However, it is very expensive to obtain reservoir data, or carry out wireline workover operations, on subsea wells using conventional techniques. For this reason, other methods are being sought which enable such alterations to be carried out more economically.

For most subsea wells the only well or reservoir data available is that obtained at the time of completion. In comparison with conventionally completed wells, reservoir management cannot be as efficient because regular monitoring and remedial work is not possible, and ironically it is in the marginal fields where efficient reservoir management can be crucial to the success of the venture.

Permanent gauges have been installed in the completion strings of some subsea wells. These enable continuous downhole pressure and temperature data to be relayed to the production installation. By this means the reservoir engineer can monitor certain aspects of the behaviour of the reservoir such as depletion, water break-through, etc. Transient tests can also be conducted as well as more elaborate interference tests to determine interwell communication.

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