In the last 20 Years four new deep wells were drilled to 
develop a scientific HDR/EGS prototype at Soultz sous 
Forêts, three old oil wells were deepened into the 
granite to observe seismicity during hydraulic 
stimulations and one additional seismic observation well 
was completely new drilled. In 1997 the upper reservoir 
or the “stimulated down hole heat exchanger” was 
tested at app. 3.5 km depth using a doublet in between 
GPK1 and GPK2. After the deepening of GPK2 and the 
drilling and stimulation of the wells GPK3 and GPK4 
the deeper reservoir at 5 km was investigated using a 
triplet with two production wells. The first step in every 
drilling operation is the choice of the right size of the 
drilling rig with a good safety margin and fitting the 
wellbore planning. During all drilling operations at Soultz 
this was the case. During times with high oil prices the 
availability of drilling rigs is not sufficient anymore, 
therefore, sometimes bigger rigs than necessary are 
used and the drilling costs are rising unnecessarily. In 
Soultz always standard oil field hole sizes were 
drilled (for example drilling 17 ½” and running 13 3/8” 
casing or drilling 12 ¼” and running 9 5/8” casing) to 
reduce the costs and to maximize the availability 
of spare parts or additional (fishing) tools.

Massive hydraulic stimulation is the key to develop and 
run an HDR/EGS power plant at economical costs 
because the natural permeability is not high enough 
within the deep granite at Soultz. Therefore the wells 
need to be able to resist the thermal cycles while 
cooling during injection and while heating during 
production. The wells at Soultz were completed with an 
uncemented internal casing which is only supported at 
the bottom by open hole casing cupper nickel 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 using  high 
temperature Aflas or Viton ring seal pack-offs. Both the 
casing packers and the wellhead seals were 
developed and improved in Soultz during the last years 
in order to obtain a usable well completion at 
reasonable cost.

The used bottom hole assemblies and drilling bits at 
Soultz were improved during the last years therefore 
longer life times at the same penetration rates 
of the used drilling equipment could be performed. 
During directional drilling with a mud motor no 
improvement of the drilling performance was observed. 
Therefore the amount of motor runs was minimized to 
the risk of the high “lost in hole” prices. The 
abrasiveness of the Soultz granites as well as the fact 
that it varies on short distances in grain size, alteration 
and number of fractures per meter PDC-bits were not 
used. The friction within the highly abrasive granite 
did not allow using integral blade stabilizers for hole 
size control. Roller reamers were used instead to 
control the well path with a slick, packed or 
pendulum bottom hole assembly. The weak point during 
directional drilling within the Soultz granite was the 
number of failures of the MWD equipment. 
While mud circulation within the granite there is only 
minor cooling of the formation observed, because the 
matrix porosity of the granite is zero. The 
combination of temperature and the vibration of the drill 
string reduced the life time of the electronic boards 
within the down hole equipment of the MWD used. 

The granite within the Rhine Graben is highly fractured 
and a natural water convection exists down to a depth 
of probably around 10 km. While drilling within these 
fractures losses and gains were observed in the drilling 
mud system. A low cost clear brine water mud was used 
to avoid losses and gains balancing the mud and 
formation fluid densities up to the second digit behind 
the Komma.

Grensasvegur 9
Reykjavik,
Iceland