ABSTRACT
Heat, fluid, and permeability are the three necessary components of all exploited geothermal fields. This chapter discusses the mechanisms for concentrating heat near the surface, the types of geothermal systems, and the environmental aspects of geothermal production. Most hydrothermal resources contain liquid water, but higher temperatures or lower pressures can create conditions where steam and water or only steam are the continuous phases. All commercial geothermal production is expected to be restricted to hydrothermal systems for many years because of the cost of artificial addition of water. Most geothermal fields are water dominated, where liquid water at high temperature, but also under high (hydrostatic) pressure, is the pressure-controlling medium filling the fractured and porous rocks of the reservoir. Geothermal energy is one of the cleaner forms of energy now available in commercial quantities. Geothermal energy use avoids the problems of acid rain, and it greatly reduces greenhouse gas emissions and other forms of air pollution.
