E. M. Zeynaly-Andabily, H. Chen, S. S. Rahman, Australian Petroleum Cooperative Research Centre, Centre for Petroleum Engineering, UNSW, Sydney 2052, Australia, and C.P. Tan, Australian Petroleum Cooperative Research Centre, CSIRO Petroleum, Melbourne 3149, Australia.

Abstract

Wellbore instability in shales is one of the primary problems in oil and gas well drilling. The problem of wellbore instability in shales has been traditionally tackled by using oil-based muds. However, this technique is costly and restricted by the Environmental Regulatory Bodies. Various non-oil-based mud approaches have been attempted based on experimental investigations. Nevertheless, there has not been an in-depth understanding of the mechanisms of wellbore instability in shales developed in these studies. Consequently, the outcome of these studies can not serve as a general guideline for formulating drilling fluids for drilling different types of shales. Recent studies have shown that borehole instability in shales can be managed by controlling the chemical potential of drilling muds. One of the critical aspects in the chemical potential approach is that shales are not ideal membrane, i.e. the reflection coefficient is less than 1. The value of the reflection coefficient of a shale being drilled must be known before one can manage the wellbore instability problem in the shale by means of controlling the chemical composition of the drilling mud. The reflection coefficient of a shale membrane varies with the type of shale being drilled, the composition of the formation water in the shale, the burial depth of the shale and the chemical composition of the drilling mud used. This paper describes a mechanistic tool which incorporates the above mentioned parameters to predict the reflection coefficient of shales. The model is verified by experimental data obtained with a shale from the North West Shelf of Australia (Shale W). An example is also given to show how the tool can be used to manage wellbore instability in shales by controlling the chemical composition of muds. The results of this study can be used as a guideline for formulating proper muds to drill troublesome shales.

Introduction

Drilling through shale may encounter a variety of problems, such as bit balling, sloughing, creep, washout and even complete hole collapse. The cost arisen from wellbore instability in shales has been estimated conservatively to be in the order of magnitude of US$500 million per year. Wells in troublesome shales have been traditionally drilled with oil-based muds. However, this technique is under critical scrutiny due to its negative environmental impact and is banned in some regions of the world. To overcome the problem, a number of non-oil-based mud approaches have been attempted. For example, O'Brien and Chenevert, Mondshine, Clerk et al. and Corley et al. developed various polymer/KCl water-based muds to improve wellbore stability in shales. Walker et al. reported the improvement of wellbore stability in shales by using potassium modified lime muds. Chenevert developed glycerol mud to reduce shale swelling. However, all these studies are based on experimental investigation only and there is no in-depth understanding of the mechanisms of wellbore instability in shales. The objective of this paper is to provide a fundamental understanding of the mechanism by which wellbore instability in shales occurs and a guideline for field engineers to manage wellbore instability problems by controlling the chemical composition of drilling muds. A mechanistic tool is provided for predicting the chemical composition of a drilling mud needed to drill a given shale safely.

Theory
Fundamentals

Borehole instability problems in shaly formations are closely connected to the "bulk properties" of shales.

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