WELL-BORE INSTABILITY IN THE NORTH WEST SHELF OF AUSTRALIA

Abstract

Well-bore instability, experienced mainly in shales, has resulted in significant drilling delays and abandonment of wells in the North West Shelf of Australia. Although these problems may be induced by physico-chemical interactions, there is an increasing awareness that instability in this region is principally associated with in situ stresses that are high relative to the strength of the materials.

This paper describes the research undertaken by the Australian Petroleum Cooperative Research Centre to assist industry in understanding and managing stress-induced well-bore instability in this region. To conduct such stability analyses the basic information required includes knowledge of the orientation and magnitude of the principal in situ stresses and the strength and deformation response of the materials to stress changes imposed by drilling. The required data can be determined using welllogging, analytical and laboratory techniques.

Analytical methods can be used to examine the relationship between well-bore stability and changes introduced through drilling. Spreadsheets based on the analytical methods have been produced and applied to the assessment of drilling alternatives and/or design of some well-bores in the North West Shelf.

The application of the critical mud weight contour plots and mud weight stability profiles produced by the spreadsheets in assessing drilling alternatives, selection of optimum well-bore alignment and mud weight design are demonstrated through examples. The analyses showed that counter to intuitive expectations, an inclined well may be more stable than a vertical well depending on the well-bore direction, deviation angle and stress regime.