Page 160 - Reservoir Geomechanics

143 Faults and fractures at depth

to it (acting to close it). This said, it is not likely that Mode I fractures affect fluid flow
because they cannot have significant aperture at depth. As illustrated in Figure 4.21,
when P f slightly exceeds S 3 , fractures of any appreciable length would be expected to
propagate, thereby dropping P f and causing the fracture to close. In fact, because the
static case in the earth is that (P f −S 3 ) < 0, transient high fluid pressures are required to
initiate Mode I fractures (as natural hydrofracs), but following initiation, the pressure
is expected to drop and the fractures to close. Hence, only extremely small fracture
apertures would be expected, having little effect on flow. Let us consider 0.3 MPa as a
reasonable upper bound for P f − S 3 in a one meter long Mode I fracture because of the
relative ease with the fracture would propagate (Figure 4.21). For reasonable values of
ν and E, equation (5.2) demonstrates that the maximum aperture of a Mode I fracture
would be on the order of 0.01 mm. Obviously, considering a fracture to be only 1 m long
is arbitrary (especially because b max increases as L increases), but as L increases, the
maximum value of P f − S 3 decreases thereby limiting b max (equation 5.2). Of course,
real Mode I fractures in rock will not have perfectly smooth surfaces so that even when
they are closed, a finite aperture will remain (Brown and Scholz 1986) such that in
rocks with almost no matrix permeability, closed Mode 1 fractures can enhance flow
to a some extent.
Faults (Mode 2 or 3 fractures that have appreciable shear deformation) are likely to
be much better conduits for flow than Mode I fractures. Figure 5.2a (modified from
Dholakia, Aydin et al. 1998) schematically illustrates how faults evolve from initially
planar Mode I fractures, sometimes called joints, or in some cases, bedding planes.
After the passage of time and rotation of principal stresses, shear stress acting on a
planar discontinuity can cause slip to occur. In cemented rocks, shearing will cause
brecciation (fragmentation and grain breakage) along the fault surface (as well as dila-
tancy associated with shear) as well as damage to the rocks adjacent to the fault plane.
Both processes enable the fault to maintain permeability even if considerable effective
normal stress acts across the fault at depth. For this reason, faults that are active in
the current stress field can have significant effects on fluid flow in many reservoirs
(Barton, Zoback et al. 1995). This will be discussed at greater length in Chapter 11.It
should be pointed out that the terms fractures and faults are used somewhat informally
in this and the chapters that follow. It should be emphasized that it is likely that with
the exception of bedding planes, the majority of planar features observed in image
logs (next section) that will have the greatest effect on the flow properties of forma-
tions at depth are, in fact, faults – planar discontinuities with a finite amount of shear
deformation.
The photographs in Figure 5.2a,b (also from Dholakia, Aydin et al. 1998) illustrates
the principle of fault-controlled permeability in the Monterey formation of western
California at two different scales. The Monterey is a Miocene age siliceous shale with
extremely low matrix permeability. It is both the source rock and reservoir for many oil
fields in the region. The porosity created in fault-related breccia zones encountered in