reservoir geomecanics

333 Wellbore stability
This type of problem is best addressed through detailed numerical analysis which is
beyond the scope of this book. This type of sophisticated analysis can incorporate
the post-failure behavior of a formation which is essential for calculating the volume
of produced sand at a given drawdown (i.e. production rate) and degree of formation
depletion. This said, there are several principles that can be illustrated using the types
of analytical wellbore failure models discussed above.
Figure 10.21a illustrates the relative stability of wells drilled at different azimuths
and deviations in the Cook Inlet in Alaska (Moos, Zoback et al. 1999). The question
addressed in that study was whether during production it might be possible to leave
multi-lateral wells uncased near the join between the multi-lateral and the main hole.
When an uncased well is put into production, the pressure in the wellbore is lower than
the pore pressure. Hence, the well is more unstable during production than it is during
drilling, somewhat analogous to underbalanced drilling.
The geomechanical model developed for this field predicted that highly deviated
wells drilled at an azimuth of ∼N30 ◦ W (or S30 ◦ E) are most stable whereas those
drilled to the ENE or WSW are most unstable. What gave this prediction added credibility
is that the drilling history of two near-horizontal wells (shown in Figure 10.21b
and pre-dating the geomechanical analysis) was revealed only after developing the
geomechanical model upon which Figure 10.21a was developed. The well drilled to
the NNW was drilled without wellbore stability problems, whereas that drilled to the
NE had severe wellbore stability problems. As such results were consistent with the
conclusions derived from the geomechanical analysis, it provided still more evidence
that the geomechanical model was correct.
Once the most stable direction for drilling was established, the next step to address
was identification of the depths at which the strongest rocks were found. These intervals
are preferred as the kick-off depths for the multi-laterals. This was accomplished
through utilizing log-based strength estimates calibrated by laboratory tests on core.
In fact, equation (5) in Table 4.1 was derived in this study. Once the most stable
drilling direction and depths were identified, the key operational question to address
was whether or not the well would remain stable as production and depletion occur
over time.
The results presented in Figure 10.22a,b address the question of stability of the
uncased multi laterals for the case of wells drilled only at the most stable depths and
in the most stable direction. Figure 10.22a shows the amount of drawdown in the
region around the well associated with a modest rate of production (∼500 psi). A finite
element analysis was used to do this calculation. After calculating the effect of the
pore pressure change on stresses around the wellbore, the stability of the producing
wellbore was calculated assuming a uniaxial compressive strength of 10,000 psi (typical
of the stronger intervals in the well). As shown in Figure 10.22b, only a modest degree
of wellbore failure is expected to result from the change in pore pressure and stress
around the wellbore associated with production. However, Figure 10.22c shows the