Permeability in high-temperature geothermal systems is dominantly fracture controlled. However, fracture characteristics and cross cutting relationships among fracture sets are generally difficult to define from cuttings samples alone. in addition, the sparse number of wells in most systems limits the data that can be obtained on the areal fracture distribution. Despite these inherent difficulties, petrologic techniques and mathematical simulations can be used to develop the models of fracture geometry, permeability, and alteration that must be addressed by reservoir engineers. Our fracture studies have taken two approaches; first, establishing predictive structural models, and, second, documenting fluid-rock interaction using alteration mineralogy and geochemistry. Examples of the use of these techniques from active geothermal fields in the U.S. and Canada are described.
Fluid flow is fracture controlled in essentially all high-temperature geothermal systems. Some data on chemical and physical conditions within the reservoir can be obtained directly from well logs and the chemistry of sampled fluids. Geological and geochemical investigations of reservoir rocks frequently can provide much additional information. This information Ray be particularly important during the early stages of a particularly important during the early stages of a geothermal program when it is difficult to obtain reliable data on subsurface temperatures and fluid chemistries because the rocks have been invaded by drilling fluids, and flow tests are generally of insufficient duration to ensure complete removal of these fluids. Later, during development of a field, when drill holes are numerous, geological and geochemical studies of the reservoir rocks can provide information on the directions of fluid provide information on the directions of fluid flow, and guide further development to areas of high permeabilities.
The purpose of this paper is to describe the application of various geological and geochemical methods to understanding fluid flow and permeability relationships in fractured geothermal systems. permeability relationships in fractured geothermal systems. These methodologies are illustrated by Iota from three systems which represent different geologic environments but which are all fracture controlled. Because of the different data bases available for these systems, our approaches to fracture mapping have been different. The Baca geothermal field in the Valles caldera, New Mexico is typical of geothermal systems associated with calderas. Meager Mountain in British Columbia is representative of geothermal systems associated with stratovolcanos. In addition, Meager Mountain displays many characteristics typical of systems associated with rhyolitic magmas emplaced in granitic terrains (e.g. Roosevelt Hot Springs, Utah; Steamboat Hot Springs, Nevada; Coso, California). Finally, the Salton Sea geothermal field provides an example of a system that has evolved provides an example of a system that has evolved from one in which fluid flow was initially through porous sandstones to a system where fluid flow is porous sandstones to a system where fluid flow is now dominantly through fractures.
The models presented here have been developed through detailed investigations of cuttings and cores combined with geologic mapping. The techniques employed include detailed lithologic logging, X-ray diffraction, electron microprobe and petrographic investigations of alteration petrographic investigations of alteration assemblages, quantitative structural modeling, fluid inclusion geothermometry, and stable isotope gas geochemistry.
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