57 Copyright © 2015, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited. Chapter 3 DOI: 10.4018/978-1-4666-8473-7.ch003 Risk Due to Wellbore Instability ABSTRACT Exploration and production as one of the most important parts of the petroleum industry encounters dif- ferent problems, usually resulting in nonproductive time and additional expenses. The most common and most expensive of them are related to wellbore instability and associated problems. Wellbore instability problems are usually related to drilling operation, but they can also appear during completion, workover, or the production stage of a certain well. The traditional solution for wellbore instability problems is composed from the early recognition of specific wellbore instability problems, the main cause identifica- tion and swift response. For more effective solution it is necessary to incorporate wellbore stability and risk assessment in the early phase of well design. This chapter gives one general overview of wellbore instability problems and their causes as well as an overview of actual approaches and methods in well- bore stability and risk assessment. INTRODUCTION The petroleum industry as well as other indus- tries, encounters different problems because of the specific nature of the job itself and always presents a possible human fault. Unlike other industries, the petroleum industry employees, especially engineers in the exploration and produc- tion department (petroleum engineers, geologists, geomechanical engineers, geophysics etc.) work in very specific conditions and environment, and with limited data availability. The main objec- tives of this part of the petroleum industry are discovering hydrocarbons reservoirs and creating facilities for their economical, safe and ecological acceptable production. One of the most important parts in the exploration and production activities is drilling a borehole, a subsurface object that connects a reservoir with the surface facility and provides a pathway for hydrocarbon production. In general, drilling is a specific technical and technological process and because of that, it requires specific engineer’s skills and the usage of specific equipment and instruments. On the other hand, drilling is very expensive. Aldred et al. in their work published in 1999 estimate that Nediljka Gaurina-Medjimurec University of Zagreb, Croatia Borivoje Pasic University of Zagreb, Croatia