Page 155 - Geothermal Energy Renewable Energy and The Environment

Drilling 141

drIllInG For GeoThermal FlUIds For power GeneraTIon
What distinguishes drilling for power generation purposes from that in any other application is the hos-
tility of the environment within which drilling must be done. Although the depth of the wells and the
mechanical challenges are similar to those in the oil and gas industry, thus allowing significant tech-
nology transfer from that community, accessing and penetrating geothermal reservoir requires special
considerations. The principal challenges that must be addressed are the high pressures and tempera-
tures of the fluid and geological environment, and the common presence of fluids that are chemically
aggressive. We will consider these challenges by first describing drilling technology in general terms
and then describe specific issues that must be addressed in order to complete a geothermal well.

drillinG riGs
The ability to drill safely and effectively in the type of environment encountered in geothermal sys-
tems requires sophisticated, state-of-the-art drilling systems that can handle the unique conditions that
will be encountered. Drilling rigs are sized according to the potential load the drill string will impose
on them. Hence, they are sized according to the depth of what they will drill to. A sense of the required
capabilities can be obtained by considering the requirements for a 6000 m well. Note, however, most
geothermal systems do not currently require drilling to such depths. Should EGS be pursued in a con-
sistent fashion, however, such drilling capabilities will be required.
A well extending to 6 km can have a drill string load exceeding 350,000 kg. Hence, the structure
of the rig must be capable of supporting that load. In addition, the system must be capable of con-
trolling the load on the bit with the appropriate force in order to achieve efficient drilling rates, as
described below. Finally, since the entire drill string must be rotated in order to drive the bit to exca-
vate the hole, the rig must be capable of applying a torque to the drill string that is on the order of
43,500 Newton-meters. The power requirements for such a system will be approximately 4.5 MW.
All drilling rigs have a tower assembly that is used to hoist individual sections of the drilling pipe
(usually between 6.1 m and 9.1 m long), hold it in place as new pipe is added to the drill string, and
to raise and lower the drill string for various aspects of the drilling process. Drill rigs differ, though,
in how they apply torque to the drill string in order to rotate the bit.
Some drilling rigs are designed to apply torque to the drill string using a rotary table. The rotary
table is located on the platform or floor of the drill rig where the pipe is lowered into the well. The
rotary table is part of the platform that clamps to the pipe and rotates the drill string at a specified
rate. Other drill rigs apply torque through what is called a top head drive assembly. The top head
drive couples to the end of the drill string and applies torque while the drill string is being lowered
into the well. Both are used in the geothermal industry.
The drilling mud capacity required for such a system must accommodate a minimum of about
135 cubic meters of capacity and the pumping capability to maintain that flow. For wells that enter
geothermal systems, it is common to include a mud cooling unit in the mud circulation system since
the return mud can be quite hot as it returns from the bottom of the well. Recirculating hot mud would
diminish the cooling capacity of the mud and reduce the lifetime bit and the rate of penetration.
Such systems are generally brought to the drill site in modular assemblies that can be erected and
disassembled relatively quickly. These are not truck mounted drilling systems. The time required to
drill such a well is described in the Kakkonda case study below.

confininG pressure and rocK sTrenGTh
One of the important effects that must be considered when drilling holes deeper than a few hun-
dred meters is that the strength of the rock increases as the confining pressure increases. Figure 8.4
summarizes how the fracture strength of a rock increases as the confining pressure increases.
Fracture strength is a measure of the stress a rock must experience before it fails by fracturing. This