Page 256 - Geothermal Energy Systems Exploration, Development, and Utilization

232 4 Enhancing Geothermal Reservoirs
700 Pressure Data 80 600 Last shut in period
Pressure at 3770m (bar) 600 Rate Reservoir pressure: 597 bar 40 Flow rate (l/s) Pressure at 3770m (bar) 560
Fit
60
650
at 3770m
20
550
500
450 0 −20 520
0 50 100 150 200 −5 −3 −1
0,5
Time (h) Superposition time (h )
Figure 4.26 Cyclic injection/production test performed in
2004. The right picture displays the pressure build up during
the last shut-in period of the test versus the square root of
time-superposition time.

The productivity of the well after stimulation was determined as 0.15 l s −1 bar −1
after 6 hours production and at a pressure level below the reservoir pressure.
Accordingly, the productivity of the well was improved due to fracturing by a factor
of 10, at least. The still limited productivity after stimulation reflects the impact of
a low permeable matrix. For a concept that is based mainly on the properties of the
fracture the productivity however, is less meaningful. In particular, the circulation
of water between two layers via this high conductive fracture, as it is envisaged in
Horstberg, will mainly depend on the fracture property and less on those of the
matrix.
It has to be emphasized that the good hydraulic properties of the created fracture
persist already over a long time. A low rate injection test performed in 2007 provided
essentially the same hydraulic parameters as derived shortly after stimulation, in
2004.

4.10.3.3 Summary and Conclusion
Waterfrac operations in two different sandstone formations of the Horstberg
well were performed after perforating the individual layers. In the
Volpriehausen-sandstone, only a small-scale operation was carried out
3
with a water volume of less than 1000 m . Here, no significant productivity
enhancement was achieved. A second formation (Detfurth-sandstone) was
3
perforated and stimulated by a much higher volume. About 20 000 m of fresh
water at a typical rate of 50 l s −1 was injected here. A huge fracture with an area
2
of more than 100 000 m was created whereof a significant part of more than
2
10 000 m retained a high (infinite) hydraulic conductivity at reservoir pressure.
Although, only a part of the original created fracture remains high conductive
after fracturing, the dimensions of this retaining fracture are still large compared
to fractures that are typically created in more permeable rock by applying the
proppant frac concept. Based on the successfully created fracture new concepts are
tested for the heat extraction from one well.