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Numerical modelling for well completion and wellbore stability

Carrie broad

Engineering Honours Degree 2005

University of Adelaide



Wellbore instability related problems have a severe impact on drilling schedule and budget. It has been estimated that around US$1 billion is lost each year due to these problems. Wellbore stability analysis has therefore been included at the planning stage of many operation companies. Mechanical induced wellbore instability can be effectively managed by determining the critical mud weights, which are mainly dependent on the in-situ stress regime and formation strength. This paper reviews the various failure mechanisms and the factors that affect wellbore stability.

It is important to determine the optimum wellbore profile and to design an appropriate mud weight program in order to prevent wellbore instability problems. The paper presents the methodology of guideline charts, which is a practical approach for the design of wellbore profiles. The charts allow assessment on wellbore drillability, optimum wellbore profile and influence of various field conditions on wellbore stability to be made with relative ease and for all drilling types.

Reservoir compaction and subsidence damage has occurred in many oil and gas fields throughout the world. When dealing with large offshore fields these problems can significantly impact on recoverable reserves where a single well can cost over $10 million. Hence, the damage or loss of even a single well can cause both technical and financial problems. There are three critical casing damage mechanisms that will be discussed in this paper. To assess more complex formation deformation and casing damage risks, geomechanical models have been developed. These include two and three-dimensional finite element type techniques. The geomechanical analysis techniques applied to assess casing damage mitigation strategies will be presented. The paper describes the commercial software FLAC3D (Itasca Consulting Group), a three-dimensional explicit finite-difference program for engineering mechanics computation.

It is the aim of this project to set up a wellbore numerical model under differing stress conditions, pore pressures, mud pressures and depth of burial, to determine the effect each has on the overall stability of the well.

Australian School of Petroleum



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