Page 253 - Geothermal Energy Systems Exploration, Development, and Utilization

4.10 Case Studies 229
3600
Salt.

Solling
Clay. 3700

Sand. Detfurth

3800 Depth (m)

Silt.

3900
Volprie-
hausen

4000
0 50 100 150
Gamma-API

Figure 4.23 Lithology and Gamma-Ray-Log in the depth range of relevance (3600–4000 m).

Detfurth After perforating the Detfurth formation, a breakdown pressure of
about 340 bar was recorded at the beginning of injection (Figure 4.24). A small
−1
injection rate of 1.7 l s , pumped for only 2 hours, was already sufficient to reach
this fracturing pressure. By including the reservoir pressure of about 230 bar
(wellhead), as determined later, the initial productivity of the Detfurth-sandstone
−1
could be estimated as lower than 0.015 l −1 sbar .This numberis anupperlimit
because the pressure was cut by fracturing and not by water diffusion. A further
reason for overestimating is its derivation after a short time period.
3
The fracture was enlarged and propagated by injecting 20 000 m fresh water
during three frac tests (Figure 4.24). The fracturing pressure was approximately
−1
330 bar at a typical flow rate of 50 l s .
It should be noted that the fracturing pressure in the Detfurth formation is
about 100 bar lower than in the Volpriehausen, only 120 m below. Obviously, the
minimum stress varies significantly over short vertical distances – an issue for
further investigations.