Core Holes

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Exploration Technique: Core Holes

Exploration Technique Information
Exploration Group: Drilling Techniques
Exploration Sub Group: Exploration Drilling
Parent Exploration Technique: Exploration Drilling
Information Provided by Technique
Lithology: Core holes are drilled to identify lithology and mineralization
Stratigraphic/Structural: Retrieved samples can be used to identify fracture networks or faults
Hydrological:
Thermal: Thermal conductivity measurements can be done on retrieved samples.
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Core Holes:
A core hole is a well that is drilled using a hallow drill bit coated with synthetic diamonds for the purposes of extracting whole rock samples from the well. These rock samples are called cores, and are very useful for characterizing lithological and structural features within a well.
Other definitions:Wikipedia Reegle


 
Introduction
Core holes are dilled to characterize subsurface lithology, mineralization, fault or fracture networks, porosity and permeability characteristics. The rock samples are very useful when attempting to characterize a geothermal system (see Core Analysis). A critical step in managing a core sample is documenting at what depth interval the core section is from, as well as preserving the sample. As soon as a core sample is brought to the surface it undergoes tremendous changes in pressure and temperature, this decrease in pressure and temperature can result in volatiles escaping from the rock mass or some minerals to rapidly undergo alteration.

Core holes are often referred to as "slim holes," with the distinction that not all slim holes recover core.

 
Use in Geothermal Exploration
Core holes are cheaper than drilling production wells and provide whole rock samples of formations at depth. However, core samples cannot be retrieved from depths greater than 2.5 km, drilling becomes more difficult to control with depth, and the wells are typically too small (3-4’’ in diameter) to use some logging or pumping tools (jennejohn, 2009).

A major advantage of core holes is that drilling is not stopped by lost circulation zones; drilling may continue through LC zones as long as fluids are pumped into the well (with resulting cost of lost fluids). This allows exploration wells to be drilled through complex stratigraphic sequences (e.g., volcanic rocks) without stopping to cure the lost circulation (Delahunty et al 2012).

Costs increase with depth over time, and deeper holes require larger initial core size (e.g., Delahunty et al 2012). Cost is also affected by drilling regulations for geothermal test wells, which may require casing string cemented in to a percentage of total depth; these rules typically require 1" cement annulus, which may require reaming of hole to attain.

 
Related Techniques
A wide range of tools exist for geophysical logging of slim holes as small as NQ size (~2 inches), including gamma, neutron, resistivity, density, and televiewer logs (Schmitt et al 2012). However, these slim hole tools are often limited to lower temperatures (<70 C), which may limit their usefulness in geothermal terrains.











Page Area Activity Start Date Activity End Date Reference Material
Core Holes At Blue Mountain Geothermal Area (Fairbank & Niggemann, 2004) Blue Mountain Geothermal Area 2002 2004


Core Holes At Fort Bliss Area (Combs, Et Al., 1999) Fort Bliss Area


Core Holes At Hot Sulphur Springs Area (Goranson, 2005) Hot Sulphur Springs Area


Core Holes At Kilauea East Rift Geothermal Area (Bargar, Et Al., 1995) Kilauea East Rift Geothermal Area 1989 1991


Core Holes At Lake City Hot Springs Area (Benoit Et Al., 2005) Lake City Hot Springs Area


Core Holes At Long Valley Caldera Geothermal Area (Benoit, 1984) Long Valley Caldera Geothermal Area 1982 1982


Core Holes At Long Valley Caldera Geothermal Area (Chu, Et Al., 1990) Long Valley Caldera Geothermal Area 1989


Core Holes At Long Valley Caldera Geothermal Area (Eichelberger, Et Al., 1988) Long Valley Caldera Geothermal Area 1987 1988


Core Holes At Long Valley Caldera Geothermal Area (Lachenbruch, Et Al., 1976) Long Valley Caldera Geothermal Area 1962 1973


Core Holes At Long Valley Caldera Geothermal Area (Urban, Et Al., 1987) Long Valley Caldera Geothermal Area 1986 1986


Core Holes At Newberry Caldera Area (Combs, Et Al., 1999) Newberry Caldera Area


Core Holes At Steamboat Springs Area (Warpinski, Et Al., 2002) Steamboat Springs Area


Core Holes At Steamboat Springs Area (Warpinski, Et Al., 2004) Steamboat Springs Area


Core Holes At Vale Hot Springs Area (Combs, Et Al., 1999) Vale Hot Springs Area


Core Holes At Valles Caldera - Redondo Geothermal Area (Fawcett, Et Al., 2007) Valles Caldera - Redondo Geothermal Area 2004 2004


Core Holes At Valles Caldera - Redondo Geothermal Area (Goff, Et Al., 1986) Valles Caldera - Redondo Geothermal Area 1984 1984


Core Holes At Valles Caldera - Sulphur Springs Geothermal Area (Gardner, Et Al., 1989) Valles Caldera - Sulphur Springs Geothermal Area 1988 1988


Core Holes At Valles Caldera - Sulphur Springs Geothermal Area (Goff, Et Al., 1987) Valles Caldera - Sulphur Springs Geothermal Area 1986 1986



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