Page 150 - Geothermal Energy Renewable Energy and The Environment

136 Geothermal Energy: Renewable Energy and the Environment

100.0

EGS

10.0

Cost (millions of $) 1.0 Hydrothermal geothermal
Oil and gas

0.1
Boreholes for
ground source heat pumps

0.01
0 2,000 4,000 6,000 8,000 10,000
Depth (meters)

FIGUre 8.1 Completion costs for drilled wells, as a function of the bottom hole depth. Data from Smith et
al. (2000). Bertani (2007), International Geothermal Association (2008) and Bolton (2009).

restricted to relatively shallow levels, the type of drilling equipment that is required is virtually
identical to that employed for drilling water wells.

drillinG equipmenT and TechnoloGy
In regions where the rock is relatively soft, such as in sedimentary basins where the subsurface is
composed of sandstone, silt, and other porous, unconsolidated material, drilling is often done with
a truck-mounted auger (Figure 8.2). An auger is a helical screw that has a hardened bit for cutting
into the soft rock or soil. As the screw rotates, it transports the drill cuttings out of the hole. Such
methods usually can reach depths of a few hundred feet in a relatively short time (hours to a few
days). Borehole sizes for these applications can be as small as 7 cm (3 inches) to greater than 25 cm
(10 inches), depending upon the design of the system and the specific application.
In regions where hard rock is expected to be encountered, it will be necessary to use drilling
equipment that can penetrate it. There are several technologies that can be employed, depending
upon the size and depth of the hole to be drilled and the budget available for the project. All of the
drilling methods excavate the borehole by shattering the rock in the immediate vicinity of the end of
the drill string. The specific technology utilized to shatter the rock, however, varies considerably.
Percussion and rotary techniques use mechanical energy to shatter the rock. Percussion drilling
shatters the rock by repeatedly hammering on it. Such systems are commonly pneumatically driven.
High-pressure air is circulated through the hole to remove rock chips and dust. Percussion drilling
is useful for relatively small diameter holes of short depth. Rotary methods are generally used for
deeper, larger holes.