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Abstract

In the period May 1991 to April 1992 Ranger Oil (U.K.) Ltd., drilled five separate wells close to salt diapir structures in the Central Graben area of the North Sea. Two different salt diapirs were appraised, one in block 23/27 and the other straddling blocks 29/2a and 22/27a.

Without exception all of the wells exceeded their planned dry hole drilling time estimates. In addition to two planned geological sidetracks, there were three further unplanned mechanical sidetracks, all caused by unstable hole conditions. A breakdown of non-productive time associated with the wells is outlined.

This paper emphasises the prerequisite of good seismic and geological data in planning salt diapir wells. The use of high angle and horizontal drilling techniques are outlined for appraisal and development, as a means to satisfy both drilling and reservoir engineering requirements in these often marginal prospects.

The history of the salt diapir drilling in other parts of the world is also reviewed. It is concluded that drilling close to salt diapirs is usually complex and that wells must be designed and managed with a high degree of flexibility.

Introduction

Salt diapirs have always been treated cautiously by operating companies because of their geological complexity which creates an unusually high degree of uncertainty in predicting lithology and pore pressure. This in turn makes the planning and drilling of wells difficult and potentially expensive.

Salt diapirs have been drilled in the North Sea since the 1970's and Ranger Oil (U.K.) Ltd., drilled a series of wells on block 23/27 based on the essential premise of staying as far away from the salt, as mapped, as possible. 'Mis strategy failed to prove commercial Quantities of hydrocarbons. Modem seismic processing techniques and reinterpretation suggested that significant reservoir potential existed further up-dip than previously mapped which led to a renewed exploration and appraisal drilling programme.

SALT DIAPIR GEOMETRY

Salt diapirs or salt domes as they are often called, are characterised by their predominant shape as illustrated in Figures 1 and 2. They play an important part in salt production, sulphur production, underground gas storage and hydrocarbon developments [1,2], the latter being the area with which this paper is concerned. Salt diapirs have no porosity or permeability and create a wide range of subtle traps such as stratigraphic and unconformity types. The traps are especially significant for petroleum exploration in highly mature areas where many of the fields around salt diapirs are economically marginal.

Salt diapirs come in a wide variety of shapes and sizes due to the diversity of salt mechanics [1] which embodies different means of movement initiation, growth dynamics and the eventual termination of growth. The diverse and complex shapes of salt diapirs means that they are difficult to describe. The size of the salt diapir can vary in diameter from 1/4 mile to 5 miles, an example of the latter being the Lake Washington Field in Louisiana [1]. The top of the salt diapir can outcrop on the surface or it can be up to 5000ft beneath the surface and the base of the salt diapir is usually between 15,000ft to 20,000ft TVD. Salt diapirs also have an anhydrite cap rock, usually 200-400ft thick, which thins out and may disappear completely on the reservoir flanks.

SALT DIAPIR LOCATIONS

Salt diapir traps are very common [1–3], on a worldwide scale and can be divided into the following categories:

* USA * North Sea * Rest of the World

USA

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