Page 138 - Reservoir Geomechanics

121 Rock failure in compression, tension and shear

In weak formations, such as chalks, grain crushing can occur at much lower pressures
than those for sandstones (Teufel, Rhett et al. 1991).
Because reservoir compaction associated with depletion is an important process in
many reservoirs, inelastic compaction is discussed in detail in Chapter 12.In addition
to porosity loss, there can be substantial permeability loss in compacting reservoirs as
well as the possibility of surface subsidence and production-induced faulting in normal
faulting environments. The degree to which these processes are manifest depends on the
properties of the reservoir (compaction will be an important factor in weak formations
such as chalks and highly compressible uncemented sands), the depth and thickness of
the reservoir, the initial stress state and pore pressure and the reservoir stress path, or
changeinhorizontalstresswithdepletion(asdescribedinChapter3).Wong,Davidetal.
(1997) demonstrated that the onset of grain crushing and pore collapse in sand reservoirs
depends roughly on the product of the porosity times the grain radius. However, in
uncemented or poorly cemented sand reservoirs, there will also be inelastic compaction
due to grain rearrangement, which can be appreciable (Chapter 12).

Tensile rock failure

Compared to the compressional strength of rock (as discussed above) and the fric-
tional strength of fractures and faults in earth’s crust (as discussed below), the ten-
sile strength of rock is relatively unimportant. The reasons for this are multifold:
First, the tensile strength of essentially all rocks is quite low, on the order of just a
few MPa (Lockner 1995) and when pre-existing flaws exist in rock (as is the case
when considering any appreciable rock volume), tensile strength would be expected
to be near zero. Second, as argued in Chapter 1, in situ stress at depth is never
tensile. As discussed in Chapter 6, tensile fractures can occur around wellbores in
some stress states because of the stress concentration at the wellbore wall. Hydraulic
fracturing is a form of tensile failure that occurs when fluid pressure exceeds the
local least principal stress. This can be a natural process, leading to the forma-
tion of joints in rock (opening-mode, planar fractures) as illustrated in the inset of
Figure 4.21. While joints are relatively ubiquitous in nature, they are unlikely to have
a significant effect on reservoir properties (such as bulk permeability) at depth because
they are essentially closed at any finite effective stress. Because fracture permeabil-
ity is highly dependent on the width of any open fracture at depth, small tensile
micro-fractures will have little influence on flow. The extension of a tensile frac-
ture also occurs during hydraulic fracturing operations when fluid pressure is inten-
tionally raised above the least principal stress to propagate a fracture which is then
filled with sand or another material as a propant to increase formation permeability
(Chapter 6).