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Abstract An analytical method, used as part of a study to develop an understanding of stress-induced wellbore instability in the North West Shelf of Australia, has been prepared as a suite of spreadsheets with macros to carry out drained and undrained stability analyses of inclined wells under a generalised plane strain condition. The contour plots of critical mud weight and failure extent produced by the spreadsheets can be used together with the mud weight stability profiles to select optimum wellbore alignment and to assist with mud weight and casing design. Introduction The value of developing fields using contemporary drilling technology to create highly deviated, extended reach and horizontal wells is realised by the operators in the North West Shelf of Australia. In this region wellbore instability has resulted in significant drilling delays and abandonment of wells, particularly in the Cretaceous shales (Reference 1). These new drilling technologies offer economic benefits through lower development costs, faster production rates and higher recovery factors. The high in-situ stresses which cause stress-induced instability in vertical wells will create a more or less severe environment for inclined wells, depending on the direction with respect to the stress field and deviation angle of the inclined wells. This is a major concern for field development as a lack of understanding and analytical design capability to manage wellbore instability represents a severe impediment to realising the full benefits offered by current and emerging inclined well drilling technology. Consequently, the Australian Petroleum Cooperative Research Centre conducted research into stress-induced wellbore instability in inclined wells to assist the industry understand and manage the instability problems. A suite of spreadsheets with macros, based on the Kirsch equations (Reference 2), has been developed to carry out drained and undrained analytical wellbore stability analyses under a generalised plane strain condition. The contour plots of critical mud weight and failure extent produced by the spreadsheets can be used together with the mud stability profiles to select optimum wellbore alignment and to assist with mud weight and casing design. Stress and Pore Pressure Distribution on the Wellbore Wall Principal Stresses P. 101^ |
