The Pailas Geothermal Field is located on the southern flank of the Rincón de la Vieja volcanic complex in northwestern Costa Rica. Since 2009, the Costa Rican Electricity Company (ICE) has drilled 7 deep directional boreholes (in addition to the 9 existing vertical boreholes). The purpose of directional drilling has been to intersect faults and permeable fractured rock layers suggested by the geothermal model proposed by the Geothermal Resources Service Center (CSRG) Geology Department (GD), and by the consulting company West Japan Engineering Consultants (WestJEC) for the Pailas Geothermal Field and thereby locate zones for production and reinjection of geothermal brine. These deep directional boreholes have been drilled from existing well pads where vertical boreholes were drilled during the Feasibility Study in which the existence of an economically exploitable geothermal reservoir was confirmed. The results of directional drilling have been positive since they have made it possible to find permeable zones interpreted as open high-angle faults that otherwise would not have been possible to find with vertical boreholes, thereby giving insight to structural evidence that supports the geological model, and incrementing the production and injection zones in the field. Lithostratigraphic correlations between directional boreholes and vertical boreholes support structural field data and permit the estimation of fault dip. Also, with directional drilling we have found a possible new deep aquifer to the west of the field that had not been observed in the other boreholes in the field and may be used as a new reinjection zone. These and other findings associated with directional drilling at Pailas Geothermal Field have allowed us to supplement the needed vapor and reinjection capacity so that in 2011 Pailas 1, the first 35 MWe Binary geothermal plant in this field, can be a reality.

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