ZUIDWAL : A GAS FIELD DEVELOPMENT WITH HORIZONTAL WELLS

Abstract

Zuidwal is probably the first gas field development with horizontal wells. Originally, a development with fifteen conventional deviated wells was planned. Three horizontal wells have been drilled instead of six deviated conventional wells.

The field is a classical structural trap situated at a depth of 1900 m., with sandstone reservoirs of 100 m. thickness subdivided into several layers, two of which have excellent reservoir characteristics.

The main motivation to drill the wells horizontally is given by the conventional analysis of pressure drop in the reservoir:

- On a deviated well the non Darcian term of the pressure drop due to the gas velocity in the vicinity of the well accounts for an important part of the total pressure drop. In a horizontal well this term is highly reduced and its productivity is much less affected by its rate.

As a result, horizontal wells will produce twice as much as the equivalent produce twice as much as the equivalent deviated well at only 20% additional cost.

The pattern of the wells consists of two horizontal sections of 100 to 200 m., drilled in the two good layers, joined together by a more and less deviated section. This profile must increase the drainage of these units and minimize eventual water coning.

Three horizontal wells have been drilled using the well-known technology of drilling and logging. The first one was completed in April 1988 and has been in production since August 1988. The two production since August 1988. The two others, drilled-in sequence, have been in production since March 1989. production since March 1989. The total lengths of subhorizontal sections of the three wells are 650 m., 540 m. and 610 m., respectively.

The first results of the wells show that the wells perform better than expected.

The main purpose of this paper is to present a review of the well behaviour present a review of the well behaviour (well testing, production logging, production history) in order to give production history) in order to give useful background on horizontal wells in a gas field.

Introduction

The Zuidwal field is one of the few accumulations of gas in the Netherlands found in the Lower Cretaceous. Its location in the middle of the Waddenzee, an inland sea, gives Zuidwal the status of an onshore field.

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