reservoir geomecanics

336 Reservoir geomechanics
Figure 10.23. The likelihood of sand production in an uncased well as a function of depletion
(expressed as reservoir pressure) and drawdown, or production rate (expressed as bottom hole
flowing pressure) for formations of variable strength (courtesy M. Brudy). For weak formations
with a uniaxial compressive strength of 1000 psi, the maximum drawdown without sand production
is over 20 MPa when the reservoir pressure is 40 MPa, but less than 6 MPa when the reservoir
pressure is 30 MPa. Hence, reducing production rate can only limit sand production in weak
formations prior to significant depletion.
found, drilling only in the most stable direction and limiting the production rate –
altogether too restrictive for efficient operations.
Figure 10.23 (courtesy M. Brudy) shows the way in which sand production in an
uncased well is related to depletion (expressed as formation pressure) and production
rate (expressed as bottomhole flowing pressure) for formations of variable strength.
For the stress conditions appropriate to this case, it is clear that in relatively weak
formations, the maximum drawdown without sand production is over 20 MPa when
the reservoir pressure is 40 MPa, but less than 6 MPa when the reservoir pressure
is 30 MPa. Stronger formations can experience appreciably more drawdown at either
reservoir pressure. It is also clear in Figure 10.23 that reducing production rates can
limit sand production in weak formations prior to depletion.
Using a finite element model of a cased, cemented and perforated well, it is possible
to consider the use of perforation orientation to prevent sand production as discussed by
Morita and McLeod (1995). Intuitively, one can see that in weak formations, perforating
at the azimuth of the minimum compressive stress would not be advisable as one would
be perforating at the azimuth where breakouts form and where the formation might
already be subjected to very high compressive stress. Figure 10.24a (courtesy M. Brudy)