Page 184 - Geothermal Energy Systems Exploration, Development, and Utilization

160 3 Drilling into Geothermal Reservoirs

Massive water frac
Gel-proppant fracs
475 m
GrSk 3/90
450 m
N 18°
288°
Shmin
E GrSk 4/05
SHmax
750 m
Figure 3.25 Well paths, stress field and direction of hydraulic fractures.

perforation of Rotliegend sandstones and a preperforated 5 in. liner for the open
hole completion of the volcanics. The high diameter of the top hole section offers
the option for installing a (corrosion-) protective casing and an ESP as well.
The super single drilling rig Streicher VDD.370.2 (www.drilltec.de) with automatic
pipe handling system has been chosen in the tendering process and started drilling
in April 2006.
Problems were encountered during the cementation of the combined production
casing 16 in. × 13 3/8 in. The frac coefficient in the open borehole section from 741
to 2381 m was known from the adjacent well to be 1.57 bar/10 m. Therefore, light
cement slurry with density of 1400 kg m −3 has been used. Already while pumping
the lead cement a sudden pressure increase just up to the gradient mentioned
above arose due to the increased flow restriction of the mud push solution (probably
loaded with cuttings) at the entrance into the smaller annulus between the 16 in.
casing and the preceding 18 5/8 in. casing. Total loss of circulation into carbonates
of the Muschelkalk (Middle Triassic) formation could not be avoided. The top of
cement detected at 1400 m was not acceptable for the production of 150 Chot
◦
thermal brine due to the resulting thermal expansion. An unconventional solution
was the top-down cementation of the annulus with a 1270 kg m −3 cement slurry
for the displacement of the 1340 kg m −3 mudremainedin the annulus. Prior
to the top-down cementation the casing has been set in tension up to the hook
load capacity of the drilling rig (370 t). So buckling of the production casing
is avoided under all possible production temperature conditions. Temperature
loggings proved the success of the ‘‘healing’’ cementation and the absorption of the
displaced fluids by the formation at the intended depth. The total loss of circulation
would had been avoided with a sufficient distance to the fracture gradient and an
immediate reaction of the operator (i.e., by reducing the pump rate).
The thick series of creeping rock salt in the Zechstein (Upper Permian) formation
have been drilled without any signs of tight hole problems due to the mud density as
−3
high as 2000 kg m . Running and cementing of the 9 5/8 in. liner went smoothly
1
◦
within the 12 / 4 in. hole with the maximum inclination being 21 (kick-off point at