Geothermal

The Geothermal Process

The first step in the geothermal electricity process is exploration. The objectives of geothermal exploration are (LUMB, J. T., 1981. Prospecting for geothermal resources. In: Rybach) to identify geothermal phenomena.

  • to ascertain that a useful geothermal production field exists
  • to estimate the size of the resource
  • to determine the type of geothermal field
  • to locate productive zones; to determine the heat content of the fluids that will be discharged by the wells in the geothermal field
  • to compile a body of basic data against which the results of future monitoring can be viewed
  • to determine the pre-exploitation values of environmentally sensitive parameters
  • to acquire knowledge of any characteristics that might cause problems during field problems.

The importance of each objective depends on a number of factors, including technology available, economics, anticipated utilization, and location.

There are many different approaches used in geothermal exploration, however the most common methods in use at this time are: geological and hydrological studies; geochemical surveys; geophysical surveys; and the drilling of exploratory wells. Geological and hydrological studies are generally the starting point of an exploration program. The basic function of these studies is to identify the location and extent of areas worth investigating in more detail, and to help recommend the best exploration methods for these areas. The information obtained during the geological and hydrological studies will be used throughout the production phase. Geochemical studies help to evaluate if the geothermal system is water- or vapor-dominated, estimating the minimum temperature expected at depth, inferring the chemical characteristics of the deep fluid, and determining the source of recharge water. The geochemical survey consists of sampling and chemical analyses of the water and gas from geothermal manifestations or wells in the study area. The surveys help to determine possible problems that may occur during the injection phase and plant utilization and how to avoid or combat them. Geophysical surveys are directed at obtaining the physical parameters of deep geological formations. There are different techniques such as seismic, gravity, magnetic, electrical and electromagnetic prospecting that help evaluate the temperature and conductivity of the reservoir parameters. They can also help to confirm the presence of geothermal fluids in the structures. The majority of these geophysical surveys are best suited to the final stages of exploration, and helping to determine the sites for exploratory wells. The final step in the exploration process is the drilling of the exploratory wells; this is the only means of verifying the characteristics of the geothermal reservoir and assessing its potential. Exploratory wells are relatively shallow and are often drilled with a coring rig, so that samples can be retrieved and inspected to determine the subsurface geologic conditions.

Once a site has been chosen through the exploration process, the geothermal wells can be drilled. Two different types of wells are typically used in the process of developing a geothermal project. Production wells are connected to the pipeline, and take steam and/or hot water to the power plant to generate the electricity. Injection wells are used to inject the fluids created during the generation process back into the reservoir.

Energy and Geosciences Institute, University of Utah. Prepared by the U.S. Geothermal Industry for the Renewable Energy Task Force (1997), Briefing on Geothermal Energy. Washington, D.C.).

(LUMB, J. T., 1981. Prospecting for geothermal resources. In: Rybach, L. and Muffler, L.J.P., eds., Geothermal Systems, Principles and Case Histories, J. Wiley & Sons, New York, pp. 77—108.)