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The Geothermal Energy Future: Possibilities and Issues 271

stimulation, and fluid injection in order to produce power. Although the project was a technical
success, in the sense that it proved that the concept of stimulating hot rock to allow circulation and
heat exchange was sound, a number of technical challenges prevented it from being a successful
demonstration of an economically viable concept. Since that time, EGS projects have been pursued
in Australia, France, Germany, Japan, Sweden, Switzerland, and the United Kingdom. Although
none of these projects is currently functioning as a commercial enterprise as an integral part of a
power grid, lessons learned from each have refined the research needs to advance EGS to commer-
cial status. Summarized in the following sections are some of the key challenges that these efforts
have identified.

drillinG and downhole equipmenT
The requirements to successfully complete a well have been discussed in Chapter 8. Several of those
requirements are particularly difficult to satisfy in the high temperature and pressure environments
encountered in EGS projects. A common problem is that of maintaining circulation and integrity
of the drilling fluid.

drilling Fluids
It is relatively common to encounter zones in the subsurface where high permeability allows drilling
fluids to escape to the surrounding rock, thus short circuiting the fluid that would cool the bit and
drill string and remove the cuttings. Materials normally used to seal such zones often have limited
ability to perform at elevated temperatures, thus making it difficult to adequately seal off such zones.
In addition, drilling muds normally used in deep wells often begin to breakdown at the high tem-
perature conditions encountered in these wells. Clays in the muds can begin to dehydrate, chemical
compounds react with other compounds in the muds, modifying the viscosity and thermal properties
of the materials, and gels can become unstable. All of these effects degrade the ability of the drilling
fluids to flow, lubricate, and seal.
high-Temperature downhole equipment
Equipment failure at high temperatures is a recurrent problem. Although the oil and gas industry
has developed technology that can operate at temperatures as high as 175°C, EGS components need
to survive temperatures of 225–250°C. This is an important challenge because equipment that can
survive in the hole at high temperatures is needed to evaluate the stress state in the rock at depth,
characterize the orientation and location of the drill string, allow steering of the bit, and provide
detailed descriptions of the subsurface conditions.
Finally, packers that can be used to seal zones for stimulation as well as to facilitate sealing of
high permeability zones that can short circuit circulation conventionally rely on elastomeric com-
pounds. Such compounds, however, have limited performance at the high temperatures of EGS
applications. New materials or methods need to be developed that satisfy the function performed
by conventional packers.

reservoir engineering
The ability to stimulate a zone using hydraulic techniques has now been well established. Controlling
the fracture properties and their geometry remains, however, an important challenge. The ability
to assess the orientation and properties of fractures that will open when stimulation is underway
needs further improvement. Measuring the orientation and magnitude of subsurface stresses at high
temperatures, as well as the properties and orientations of existing fracture sets needs refinement.
As stimulation proceeds, it is also important to be able to control the rate of fracture growth through
knowledge of the likely response of the rock mass to changes in pressure, pumping rate, and fluid
properties. Accomplishing these goals with a high degree of certainty is currently beyond avail-
able technology. Although the scientific principles involved in rock fracture mechanics are well