Page 147 - Reservoir Geomechanics

130 Reservoir geomechanics

Healy (1984) based on stress measurements from much shallower depths. It should be
noted that Townend (2003) pointed out that the uncertainty estimates in Figure 4.26 are
likely significantly smaller than those shown.
There are two implications of the data shown in Figure 4.26. First, Byerlee’s law
(equation 4.41), defined on the basis of hundreds of laboratory experiments, appears to
correspond to faults in situ. This is a rather amazing result when one considers the huge
difference between the size of samples used for friction experiments in the lab and the
size of real faults in situ, the variability of roughness of the sliding surface, the idealized
conditions under which laboratory experiments are conducted, etc. Second, everywhere
that stress magnitudes have been measured at appreciable depth, they indicate that they
are controlled by the frictional strength of pre-existing faults in the crust. In other words,
the earth’s crust appears to be in a state of failure equilibrium and the law that describes
that state is simple Coulomb friction, or Amontons’ law as defined in equation (4.39).
In fact, we will find that this is the case in many sedimentary basins around the world
(Chapters 9–12).
In shaley rocks, it is widely suspected that the coefficient of friction may be sig-
nificantly lower than 0.6, especially at low effective pressure. In fact, Byerlee pointed
out that due to water layers within its crystallographic structure, montmorillonite has
unusually lower frictional strength because intracrystalline pore pressure develops as
it is being deformed. This manifests itself as a low friction. In recent drained labora-
tory tests Ewy, Stankowich et al.(2003) tested a deep clay and three shale samples
and found coefficients of friction that range between 0.2 and 0.3. The subject of the
frictional strength of shaley rocks is complicated, not only by the issue of pore pressure
butby the fact that many tests reveal that clays that have low frictional strength at low
effective pressure have higher frictional strength at higher effective pressures (Morrow,
Radney et al. 1992; Moore and Lockner 2006).

Limits on in situ stress from the frictional strength of faults

Because earth’s crust contains widely distributed faults, fractures, and planar discon-
tinuities at many different scales and orientations, it is self-evident that stress magni-
tudes at depth (specifically, the differences in magnitude between the maximum and
minimum principal stresses) will be limited by the frictional strength of these planar
discontinuities. Building upon the arguments of the previous section,we demonstrate
below how the frictional strength of pre-existing faults in the crust limits the possible
range of stress magnitudes at any given depth and pore pressure. While the observa-
tion that the stress magnitudes in the crust in many areas are in equilibrium with its
frictional strength (see Chapter 1) enables us to make specific predictions of stress
magnitudes at depth, we will not assume that this is always the case. Rather, we will