Page 297 - Reservoir Geomechanics

275 Stress fields

essentially perpendicular to the strike of the normal faults in the region (Stock, Healy
et al. 1985), as expected for a normal faulting regime (Figure 5.1b).
When pore pressure is sub-hydrostatic, normal faulting occurs at a value of least
principal stress that is lower than would be found at higher pore pressures. Because
pore pressure is significantly below hydrostatic in this part of the Nevada Test Site,
the process of drilling the well (and filling the borehole with drilling fluid) caused
drilling-induced hydraulic fracturing and lost circulation to occur. Following drilling,
the drilling-induced hydraulic fractures were observed on borehole televiewer logs
(Stock, Healy et al. 1985)as shown in Figure 9.4b. Because this well was continuously
cored, it is known that the fractures shown were not present in the formation prior
to drilling. Apparently, all of the drilling fluid (and cuttings) went out into hydraulic
fractures (such as the one shown in Figure 9.4b) as the hole was being cored. The
explanation of this phenomenon is quite straightforward. Thus, raising the fluid level in
the hole during drilling causes the pressure at ∼600 m depth to exceed the least principal
stress and induces hydraulic fracturing. Hydraulic fracture propagation occurs when the
fluid height in the borehole is ∼200 m below ground level. Stock, Healy et al.(1985)
showed that the same phenomenon can be seen in three different holes drilled in this
area. Interestingly, these wells could be successfully cored with 100% lost circulation.
In east Texas, another area characterized by active normal faulting, measurements
of the least principal stress made in various lithologies of the Travis Peak formation
are also consistent with frictional faulting theory. The abscissa of Figure 9.5 is the
measuredleast principalstressvalueindifferentlithologiesinthestudyareawhereasthe
ordinate is that predicted by equation (4.45) for the appropriate depth and pore pressure
(and coefficients of friction of ∼0.6). Note that regardless of whether the formation
is sandstone, shale, siltstone or limestone, frictional faulting theory incorporating a
coefficient of friction of 0.6 accurately predicts the measured stress values over a
significant range of stresses.
Normal faulting is also seen in the central graben area of the North Sea. Figure 9.6
shows least principal stress values at the crest of the Valhall anticlinal structure (mod-
ified from Zoback and Zinke 2002). In this figure, the measured value of the least
principal stress is shown at various pore pressures as depletion occurred over time (the
approximate date of the measurements is also shown). Note that the measured values
of the least principal stress in this weak chalk reservoir is predicted well by frictional
faulting theory with a coefficient of friction of 0.6 (the solid line passing through the
data) obtained from equation (4.45). The importance of normal faulting for formation
permeability is discussed in Chapter 11. The evolution of the state of stress with deple-
tion in the Valhall field (and faulting on the flanks of the reservoir induced by depletion)
will be revisited in Chapter 12.However, it is important to note that for all the rock
types in the case studies considered so far, the fact that a coefficient of friction of ∼0.6
is applicable is still further support for the applicability of this friction coefficient to