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Title: The Hawaiian Scientific Observation Hole (SOH) Program: Costs and History of a Successful Slim Hole Drilling Program 
Author: Olson, Harry J.; Deymonaz, John E.
Date: 1993
Publisher: Geothermal Resources Council
Citation: Olson HJ, Deymonaz JE. 1993. The Hawaiian Scientific Observation Hole (SOH) Program: Costs and history of a successful slim hole drilling program. Geothermal Resources Council Transactions. 17: 443-450.
Abstract: To assess the geothermal potential of the KERZ, a fence of four holes, three of which were drilled, were sited along the long axis of the KERZ within existing Geothermal Resource Subzones. These holes were drilled by a Universal 5000 core/rotary drilling rig, and were located to provide stepout drill coverage between existing and planned geothermal production wells, and to pair the SOHs with production wells to test for permeability across the rift zone. Successful drilling techniques and casing procedures were devised as the rock section became known and its characteristics noted. Above 270°F a complex stearate was added to the drilling fluids to maintain lubricity. Above 330°F a mixture of soda ash, high temperature polymer, complex stearate, and sepiolite virtually eliminated the high torque and vibration problems frequently associated with high temperature drilling. The core and other data from the SOHs have proven to be extremely valuable for both active developers in siting production wells, and in the understanding of the subsurface geologic conditions. The first hole drilled, SOH-4 provided thermal and permeability conditions along the eastern portion of the True/Mid-Pacific Geothermal Venture’s lease, and was instrumental in the proposed location of True’s #2 site. SOH-4 was drilled to a total depth of 6,562 feet in 151 days at a direct drilling and testing cost of $1,466,848, and recorded a temperature of 583°F at a depth of 6,400 feet. The second hole, SOH-1, effectively defined the northern extent of the Puna Geothermal Venture’s (PGV) HGP-A/PGV reservoir, doubled the proven reservoir size, and provided sufficient data to the lending institution for continued project funding. SOH-1 was drilled on a PGV lease to a total depth of 5,526 feet in 217 days, which included 6-1/4 days standby waiting on administrative permits, at a direct drilling and testing cost of $1,643,544, and recorded a bottom hole temperature of 403°F. The third hole, SOH-2, also was drilled on a PGV lease to a total depth of 6,802 feet in 126 days at a direct drilling and testing cost of $1,106,684, recorded a bottom hole temperature of 662°F, and may have intersected a potential reservoir at a depth of approximately 4,900 feet.
Series/Report No.: School of Ocean and Earth Science and Technology contribution number 3190.
Pages/Duration: 8 p.
Keywords: SOH, Puna, slim hole, testing

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