reservoir geomecanics

396 Reservoir geomechanics
can also be differential pipe sticking due to the difference between the mud weight and
pore pressure in the depleted formations and there could be considerable invasion and
formation damage in the depleted formation if further production was planned for the
depleted reservoir. If much lower mud weights are used to avoid these problems in the
depleted zone, wellbore instability could be a significant problem above and below it.
As discussed by van Oort, Gradisher et al. (2003) there are a variety of techniques
that can be used to address the problem of drilling through depleted formations.
Some of these are related to the use of water-based mud and lost circulation additives
discussed in the section of Chapter 10 addressing drilling with mud weights above the
fracture gradient. As was discussed in Chapter 10,itcould also be advantageous to drill
in optimal directions to avoid hydraulic fracturing near the wellbore and lost circulation
when drilling with mud weights above the least principal stress. Other techniques
include the use of additives to prevent mud penetration into the formation (see also
Reid and Santos 2003) and the use of formation “strengthening” additives which, in
effect, cements the grains of the formation out in front of an advancing wellbore thus
making drilling easier (see also Eoff, Funkhauser et al. 1999 and Webb, Anderson et al.
2001).
While drilling through depleted reservoirs can be considerably more problematic than
drilling through the same reservoirs prior to depletion, hydraulic fracturing in depleted
reservoirs can be easier than prior to depletion (and surprisingly effective if stress
rotation has accompanied depletion). The various papers presented in the compilation
of Economides and Nolte (2000) discuss many different aspects of reservoir stimulation
using hydraulic fracturing. It is worth briefly discussing the advantages of repeating
hydraulic fracturing operations (or re-fracturing) depleted reservoirs, two topics not
considered by the papers in that compilation.
One type of reservoir that would be particularly advantageous to consider hydraulic
fracturing after depletion is those in which significant rotation of principal stress directions
occurs. The conditions under which such cases are likely to occur were discussed
in the previous section. Clearly, if hydraulic fracturing was used when wells were
initially drilled in a tight reservoir, rotation of principal stress directions during depletion
would cause any new fracture to propagate at a new azimuth, possibly accessing
previously undrained parts of the reservoir.
A second advantage of hydraulic fracturing a depleted reservoir is illustrated in Figure
12.10.Incases in which there is only a small contrast in the magnitude of the least
principal stress between the reservoir and caprock prior to depletion (Figure 12.10a,
modified after Wolhart, Berumen et al. 2000), it is difficult to extend a hydraulic fracture
far from a well without the potential for vertical hydraulic fracture growth. This is
illustrated by the fracture growth simulation in Figure 12.10c. It is important to avoid
vertical fracture growth because of the potential of connecting to water-bearing strata.
Hence, reservoirs in which there is only a small contrast in the magnitude of the least
principal stress between the reservoir and adjacent formations may be poor candidates
for hydraulic fracturing, or at least limit the degree to which hydraulic fracturing can