Slimhole exploration is usually done because in many circumstances it is cheaper than rotary drilling in the same reservoir, but there are other significant benefits to having complete core samples over the depth of the hole.
Because geothermal reservoirs produce primarily through fractures, it is extremely useful to know the fracture density, spacing, and aperture, the combination of which gives a measure of the reservoir’s transmissivity. The fractures’ orientation (dip and strike) gives information on how production wells should be directionally drilled in order to intersect the maximum number of fractures. This information about fractures can be obtained from imaging logs in production- sized holes, but the logging is expensive and the instruments are often limited by the temperatures that they can withstand. Wireline coring is routinely done at temperatures well
above 200oC, and the core can be oriented with a relatively simple method, given that the hole inclination is more than 5O from vertical. Although these measurements are not always required, core samples can also provide rock properties and an opportunity to look for thermal alteration in the formation.
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