Dennis L. Nielson1
(1) University of Utah, Salt Lake City, UT
Abstract: Fractured granite geothermal reservoirs
Many of the world's high-temperature geothermal reservoirs produce water and steam from faulted and fractured granitic rocks. Economic reality requires that wells into these systems produce very high volumes of fluids; production rates are often well-bore limited. Although storage is confined to fractures, faulting appears to be responsible for high permeability fluid flow paths. Granitic host rocks are excellent reservoirs because of their brittle nature and resistance to subsequent alteration. A large percentage of well-studied granite-hosted geothermal systems display permeability that follows multiple, often orthogonal, orientations. Examples include The Geysers and Coso Hot Springs in California and Roosevelt Hot Springs in Utah. All high-temperature geothermal systems are driven by magmatic heat sources. During their evolution, the systems will cool allowing meteoric fluids to circulate to greater depths. Eventually, the fluids penetrate the crystallized heat source, and, to that extent, all high-temperature systems will at some point in their lives be hosted by a granitic rock. Most geothermal operators have adopted the procedure of directionally drilling wells to improve production. Appropriate assessment techniques include seismic monitoring, borehole imaging and chemical tracers.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana