reservoir geomecanics

324 Reservoir geomechanics
and S hmin .Asimilar problem occurs when drilling through a highly depleted sand in
order to reach a deeper horizon because of the decrease in the magnitude of the least
principal stress accompanying depletion (see Chapter 12).
In this section, we follow Ito, Zoback et al. (2001) and address the theoretical possibility
of drilling with mud weights in excess of the least principal stress for cases
of particularly high pore pressure (or high minimum mud weights needed to maintain
wellbore instability). In fact, there is empirical evidence that this can occur. The
following was reported by a major oil company drilling in the Gulf of Mexico:
While drilling a highly deviated well at elevated pore pressure, lost circulation occurred at an ECD
of 14.8 ppg caused by a pressure surge while attempting to free a stuck logging tool. The measured
value of the least principal stress at this depth was 13.0 ppg. Lost circulation material (LCM) was
used to establish circulation at 14.9 ppg (1.9 ppg over the least principal stress). Once circulation
was re-established, the well drilled to TD with an ECD of ∼14 ppg (1 ppg above the measured least
principal stress).
We will return to this anecdotal account and offer one explanations of why it was
possible to re-establish circulation and continue drilling with mud weights greater than
the least principal stress.
We consider three critical wellbore pressures, p frac , p link and p grow .For the general
case of a well that is deviated with respect to the in situ stress field (discussed in Chapter
8), tensile fractures initiate at the wellbore wall at a pressure we will call p frac .Asthese
fractures grow away from the wellbore wall they will attempt to turn to be perpendicular
to the least principal stress and link up at p link . Once the fractures have linked up and
turned to be perpendicular to the least principal stress, they propagate away from
the wellbore at p grow .Itisobvious that lost circulation cannot occur if the wellbore
pressure during drilling is below p frac .However,evenifp frac is exceeded and tensile
fractures initiate at the wellbore wall, fracture propagation (and hence lost circulation)
will be limited as long as the wellbore pressure is below p link , the pressure required
for multiple tensile fractures to link up around the wellbore. Finally, if the wellbore
pressure is greater than p link , the fractures will not grow away from the wellbore (and
significant lost circulation will not occur) if the wellbore pressure is below p grow , which
must exceed (if only slightly) the least principal stress. In general, our modeling shows
that p frac and p link can be maximized by drilling the wellbore in an optimal orientation,
and p grow can be maximized by using “non-invading” drilling muds that prevent fluid
pressure from reaching the fracture tip.
First, let us consider p grow , the fluid pressure in the fracture necessary to cause fracture
propagation once it has already propagated away from the wellbore. For simplicity, the
fracture is modeled as a penny shaped fracture oriented normal to S 3 . The pressure
distribution in the fracture is assumed be uniform, as shown in Figure 10.16, which
means that if there is a significant pressure gradient in the fracture, we will be calculating
alower bound value of p grow .However, we must take into account the fact that drilling