Since its beginning in 1987, the Soultz Project was designed to validate a modified
HDR / EGS concept associating an attractive geothermal gradient with the natural
permeabilities and formation fluids expected at depth in a Graben setting. In such a
geological setting a dense fracture network intersected by major tectonic features
can be expected. The results of a drilling operation are reported here which involved
reaming and drilling of hot and fractured granites using a low cost clear brine water
mud. Highly permeable zones were temporarily (to be available later for scientific
investigations) sealed spotting pills of loss circulation material (LCM). Experience
in Soultz indicated that the effectiveness of LCM pills under geothermal conditions
(bottom hole temperatures 200 °C) can be increased considerably by giving the heat
sufficient time to act on the mixture before resuming drilling. The well was
completed with an uncemented 9 ⅝" internal casing (0   4546 m) which is only
supported at the bottom by open hole casing packers and a short cementation. Thermal
expansion and contraction, of the otherwise self-supporting pipes (during injection
and production experiments), are compensated in the well head assembly. To our
knowledge, the well reported here (GPK3) was the first directional well with such a
completion. Drilling operations were finished after 142 days, some 16 days ahead of
schedule.

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