X. Chen, SPE, Australian Petroleum Cooperative Research Centre, CSIRO Petroleum & Monash University, Melbourne, Australia; C.P. Tan, SPE, Australian Petroleum Cooperative Research Centre, CSIRO Petroleum, Melbourne, Australia; and C.M. Haberfield, Monash University, Melbourne, Australia

Abstract

Mechanically-induced wellbore instability can be managed by determining the critical mud weights that provide sufficient wellbore wall support to counteract the redistribution of stresses resulting from the creation of the wellbore. The critical mud weights are mainly dependent on the in-situ stress regime, in-situ pore pressures, wellbore direction and inclination, and formation properties and drainage conditions. In this paper, a review of the various failure mechanisms and the effects which the mechanical factors (attributes) have on wellbore stability are presented. The review includes a summary of the typical ranges of the key attributes as determined from the literature. A series of sensitivity analyses which demonstrate the influences of these attributes on wellbore stability are presented and discussed. The analyses are based on shale properties and in-situ stress regimes typical of the North West Shelf of Australia. Finally, guidelines for wellbore stability analysis for practical well design are described.

Introduction

In the current climate of deeper reservoir exploration and increased exploitation of offshore reservoirs in the world's sedimentary basins, costs of production are significant. In Australia, nearly 90% of petroleum reservoirs are found offshore. The cost of drilling a well in such an environment can be up to A$10 million, so the chances of commercial success are not high. However, an acceptable profit margin can still be achieved by drilling directional wells from a central platform so that the number of drilling platforms can be reduced significantly. This incentive of reducing investment costs has led to the continuing development and application of directional drilling technology. However, one of the main potential problems associated with the application of directional drilling is wellbore instability. Such instability results in not only additional cost but also significant drilling delays and abandonment of wells. An estimate of US$500 million is lost each year worldwide, directly or indirectly caused by wellbore instability. Therefore, in order to fully obtain the benefits of the directional drilling technology, wellbore stability analysis has been of increasing demand in the planning stage of the wells.

Nearly all wellbore instability problems occur in the weaker rock formations, predominantly shales. The awareness of high-risk shale formations has led to considerable research on shale mechanics, which involves either chemical or mechanical investigation or a combination of both. Although many instances of instability result from a combination of both mechanical and chemical instability, mechanical factors play a dominant role in wellbore instability during the drilling phase of operations. For example, borehole instability is observed even with the most inhibitive drilling fluids, e.g. oil-based mud. Also, mechanically-induced instability caused by high in-situ stresses in vertical wells can create a more or less severe environment for inclined wells, depending on the direction and inclination of the wells with respect to the stress field. Significant effort, therefore, has been put into mechanically-induced instability studies.

In this paper, firstly, a review of the various failure mechanisms and the factors (attributes) which affect wellbore stability is presented. The review includes a summary of the typical ranges of these key attributes as determined from the literature. A series of sensitivity analyses that demonstrate the influences of these attributes on wellbore stability are then presented and discussed. The analyses are based on shale properties and in-situ stress regimes typical of the North West Shelf of Australia. Finally, guidelines for wellbore stability analysis for practical well design are described.

Wellbore Failure Mechanisms

Drilling a well in a formation changes the initial stress state and causes stress redistribution in the vicinity of the wellbore. The redistributed stress state may exceed the rock strength and hence, failure can occur.

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