Page 269 - Geothermal Energy Renewable Energy and The Environment

Use of Geothermal Resources: Economic Considerations 259

can be obtained for these parameters through discussions with those who have worked in the area,
particularly geologists and drillers.
Although the preliminary resource assessment will require collecting field data, the magnitude
of that effort can be minimized by conducting a thorough review of the existing scientific and tech-
nical literature. Literature searches of scientific journals, state geologists reports, U.S. Geological
Survey reports, historical records regarding mineral claims, and so forth, can determine what addi-
tional data should be collected, what type of data it should be, and where it should be collected.
The process of developing a preliminary resource assessment is equivalent to constructing a
conceptual model of the geological system. This conceptual model will act as an hypothesis that
will guide development of a testing program to evaluate the validity of the model and allow it to
be refined. To do this requires data from the subsurface. The preliminary resource assessment thus
becomes the basis for designing a drilling program to test and refine the initial conceptual model.
Development of a preliminary resource assessment should require less than a year.

refininG The resource assessmenT ThrouGh exploraTion drillinG
A drilling program designed on the basis of a good preliminary resource assessment will target
locations that are likely to have high geothermal gradients. How highly resolved the target area will
be depends upon the level of detailed information generated during the preliminary assessment.
Regardless of the resolution, it is common that at least two exploration holes will be drilled. More
holes will be necessary if the goal of an assessment is to define the extent of a geothermal resource.
Once target sites have been selected and prepared and target depths established, a drilling rig
that has the capability to reach the necessary depths and can handle the expected conditions is
deployed. Usually these services are obtained through contract.
The holes that will be drilled vary in type and purpose. Slim holes, with diameters of 7.5 cm
to 15 cm, can be drilled to depths of several thousand meters. The purpose of such holes is to
inexpensively access depths where temperature gradients can be measured. These measurements
allow significant refinement of the preliminary resource assessment by reducing the uncertainty
associated with estimates of the temperature, extent, and depth of the reservoir.
Cuttings from these wells or larger diameter wells provide samples of the geological rock units
in the subsurface and their characteristics. Particularly important is the porosity and permeability
of the reservoir rocks, and the extent to which permeability is fracture controlled. This information
allows refinement of estimates of likely flow rates.
If it is possible to allow the wells to stand for a period of time, it may be possible to obtain rela-
tively uncontaminated fluid samples. Chemical analysis of the collected fluids can reduce uncer-
tainty about the chemical properties of the reservoir fluid and help determine if mitigation strategies
will be required for using or disposing of the geothermal fluid.
Depending on the number of wells and the complexity of the geology, it may take up to a year to
complete a refinement of the resource assessment.

producTion wells and feasibiliTy sTudy
At this point, the resource assessment has concluded that there is an accessible resource suitable to
meet the needs of the project. The next step is to drill a production well into the reservoir in order
to establish that flow rates, temperatures, and other operational conditions are suitable. Drilling
production wells often require satisfying additional permitting and regulatory requirements. Any
such requirements should be known and in the process of being satisfied before termination of
the exploratory drilling program, if that drilling program has provided positive indications of a
usable resource.
The characteristics of production wells were discussed in Chapter 8. Undertaking this part of
the project is perhaps the most challenging, since it is at this point that the success of a project is