In this chapter I address a number of problems related to wellbore stability that I illustrate through case studies drawn from a variety of sedimentary basins around the world. In this chapter we focus on wellbore stability problems associated with mechanical failure of the formations surrounding a wellbore. Failure exacerbated by chemical reactions between the drilling mud and the formation is addressed only briefly. I make no attempt to discuss a number of critically important issues related to successful drilling such as hole cleaning, wellbore hydraulics, mechanical vibrations of the drilling equipment, etc. and refer readers to excellent texts such as that of Bourgoyne Jr., Millheim et al. (2003).

In each case study considered in this chapter, a comprehensive geomechanical model was developed utilizing the techniques described in previous chapters. The problems addressed fall into two general categories: Preventing significant wellbore instability during drilling and limiting failure of the formation surrounding the wellbore during production. The latter problem is sometimes referred to as sand (or solids) production as significant formation failure during production results in fragments of the formation being produced from the well along with hydrocarbons. Another aspect of wellbore failure with production, the collapse of well casings due to depletion-induced compaction and/or the shearing of wells by faults through injection- (or depletion-) induced faulting, will be discussed in Chapter 12.