reservoir geomecanics

10 Wellbore stability
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.
When considering the types of wellbore stability problems that could occur while
drilling, we must first define what we mean as a stable well and an optimal mud weight.
The practical manifestation of this is related to the concept of the safe mud window for
drilling, a term referring to the difference between the minimum and maximum mud
weight one should use when drilling at given depth. If wellbore stability is not a concern
in a given area, the minimum mud weight is usually taken to be the pore pressure so
that a well does not flow while drilling. When wellbore stability is a consideration, the
lower bound of the mud window is the minimum mud weight required to achieve the
desired degree of wellbore stability. In both cases, the upper bound of the mud window
is the mud weight at which lost circulation occurs due to hydraulic fracturing of the
formation.
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