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142 Geothermal Energy: Renewable Energy and the Environment

2000

Granite
1500 Basalt
Fracture strength (MPa) 1000 Sandstone

500 Dolomite
Marble

0
0 100 200 300 400 500 600 700
Confining pressure (MPa)

FIGUre 8.4 Rock strength as a function of rock type and confining pressure. (Curves are based on data
in Lockner, D. A., Rock Physics and Phase Relations, American Geophysical Union. Handbook of Physical
Constants, vol. 3, 127–47, 1995.)

parameter provides a means for gauging the difficulty a drill bit would have grinding its way into the
subsurface. Two points are particularly evident in the figure. First, the rock-type has a strong influ-
ence on the strength of the rock. Even at low confining pressure (<100 MPa), the fracture strength
varies by a factor of three. Second, as the confining pressure increases, the fracture strength of the
rock increases, but by different degrees depending upon the rock type. For geological environments
where the geology is complex, consisting of different types of rocks that are interlayered, drilling
rates will vary and the energy required to drill a deep hole will vary.
The impact of this variation in rock strength is clear when one considers how drilling rate varies
with rock type and with increasing confining pressure. Figure 8.5 shows several examples of how
drilling rate is affected by rock type and confining pressure. Note that drilling rate can decrease by
as much as a factor eight over a relatively short depth interval, with most of the change occurring
within the first ~15 MPa (~500 m) pressure increase. This effect must be considered when schedul-
ing drilling activities—for relatively deep holes, it is not unreasonable to allocate ~50% of the drill-
ing time to completing the bottom ~10%–~20% of the hole.
A technique employed to accomplish deep drilling is to increase the weight on the bit, which will
directly translate into greater force applied to the rock to overcome the rock strength. Figure 8.6
shows how the rate of penetration into the rock varies with the load placed on the bit. Penetration
rate can be increased by up to a factor of two by approximately tripling the load on the bit. However,
Figure 8.6 also emphasizes a crucial aspect that affects penetration rate, and that is the nature of the
fluid that is used to circulate in the drill hole.
The highest drilling rates obtained in the experimental study shown in Figure 8.6 were obtained
using water as the cooling agent. However, water is inadequate for clearing cuttings from the hole—
higher viscosity fluids are required to accomplish that. The other fluids used in the study were
various mixtures using mineral oil and water, or other compounds. This figure emphasizes the
importance of selecting drilling fluids that will minimally diminish penetration rate. Selecting a
drilling fluid, however, remains a challenge, as discussed below.
Rock strength also impacts the ability to maintain the desired orientation to the well. In holes
that are intended to be vertical, it is not uncommon for some unintended deviation to occur. This
results from the variability in rock strength that can occur on the submeter scale due to the inherent