| Title | Fracture Distribution in Slimholes Drilled for Project Hotspot: The Snake River Geothermal Drilling Project and the Implications for Fluid Flow |
|---|---|
| Authors | Kessler, James; Evans, James P. |
| Year | 2011 |
| Conference | Geothermal Resources Council Transactions |
| Keywords | Fracture characterization; Wireline logs; Permeability; Deep slimhole core; Geothermal |
| Abstract | Project Hotspot: The Snake River Geothermal Drilling Project is an effort to determine the potential for geothermal energy development in the central Snake River Plain, Idaho. Few data regarding the lithologies or distribution of fractures in the vertical profile are available in the central Snake River Plain so drilling provides key data in the vertical profile to help determine the geothermal potential. Three slimhole boreholes are drilled at the Kimama, Kimberly, and Mountain Home sites to depths around 2000 m. Observations of the core will be compared with data in the complete suites of borehole geophysical logs acquired in the holes. The direct observation of lithology and fractures will be used as a constraint on geophysical data to confirm interpretations of mechanical and flow properties of the rocks. Final analysis will then describe the implications for fluid flow. Thin section and permeability test results will be used to evaluate the changes in potential fracture flow characteristics with depth. This effort will provide direct observation of samples at depth and will provide an example of a workflow to connect rock core properties, wireline log responses, and flow properties. Wireline logs can help us understand basic rock properties and the relationships between fracture distribution and rock type. Critical to the development of geothermal systems is the porosity and the potential for fluid flow in the subsurface. The volcanic nature of the formations means there is little primary permeability to contribute to fluid flow. Fractures are the primary mode of fluid transport so the analysis of fractures and the level of saturation in fracture zones are necessary to evaluate the geothermal potential. Preliminary analyses of the core show that fractures exhibit a series of alteration phases, including amygduloidal fine-grained calcite. Despite the nearly zero porosity of the matrix in the rocks, mineralizing fluids appear to form connected pathways in the rock. The borehole geophysical data show where fracture zones exist and whether the porosity produced by the fractures is sufficient for geothermal development. |