Fluid Inclusion Stratigraphy: A New Method for Geothermal Reservoir Assessment - Final Project Report

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Fluid Inclusion Stratigraphy: A New Method for Geothermal Reservoir Assessment - Final Project Report - Final Project Report. (PDF file, 106 pages, 7.7 megabytes).


Publication Number: CEC-500-2013-064
Report Date: March 2007



Fluid inclusion stratigraphy is a method for identifying fractures and fluid types in geothermal systems. Open fractures are needed to provide pathways for the migration of fluids (gases, hot geothermal water, or cooler waters) within the geothermal reservoir. Fluid inclusions sometimes occur in crystals contained in rock cuttings from geothermal wells. These fluids contain gases whose geochemistry can be analyzed and plotted in well logs and subsurface diagrams to show the distribution of different chemical components within portions of a geothermal reservoir. This study found that the relative proportions (ratios) of certain gaseous components can indicate zones of cool groundwater inflow, warm geothermal fluid flow and permeable and nonpermeable zones within the geothermal reservoir. Permeable zones, which allow the easy movement of liquids and gases, are indicated by a large change in the relative concentrations of carbon dioxide and nitrogen. Different fluid types in the inclusions can be recognized based on the occurrence of certain chemical compounds, such as water, and by the ratios of certain other compounds or elements such as nitrogen argon, carbon dioxide methane, and others.

Several subsurface diagrams and well logs were developed for the Coso geothermal reservoir in Inyo County, California, showing the distribution of fluid types by depth, and indicating the general sources and temperatures of origin of the fluids. This allowed recognition of critical areas within the reservoir that can be related to zones where fractures permit higher rates of fluid flow and higher temperatures of the geothermal fluids. Significant differences between the reservoir’s western and eastern portions were found, corresponding to areas of increased permeability and present-day geothermal production. By combining the location of the reservoir fluids and permeable zones, likely production zones can be targeted in the future, potentially making reservoir management easier and less expensive.