Sidetracking Experiences in Hot Granitic Wellbores

Abstract

In the development of the first Hot Dry Rock (HDR) geothermal energy extraction systems at Fenton Hill, west of Los Alamos, New Mexico, manmade reservoirs were created by connecting two holes in hot, impermeable crystalline rock with hydraulically produced fractures. This system consists of two near vertical, 24.5 cm diameter holes approximately 3 km deep in Precambrian basement rock, at a bottom hole temperature of 200°C. In order to improve the connection between the wellbores, the production hole was sidetracked to intercept the fracture zone at a more favorable depth. Two successful sidetrack operations were accomplished in 1977, utilizing cement plugs, underreaming, Dyna-Drills, and both button and diamond bits. Drilling of the second larger, commercialized reservoir system began in1979 and consists of two boreholes drilled to a depth of 4 km at an angle of 35° from the vertical, which will be connected by a series of hydraulic fractures extending across the 400 m vertical separation of the two holes. Sidetracking to bypass a stuck bottom hole assembly was accomplishes through the use of a whipstock device, Dyna-Drills, and button bits. This paper is presented as a case history of the efforts involved to achieve successful sidetracking in hot granitic wellbores.