Page 254 - Geothermal Energy Systems Exploration, Development, and Utilization

230 4 Enhancing Geothermal Reservoirs
400 250
Breakdown
350 1. Frac 2. Frac 3. Frac 200
Wellhead pressure (bar) 300 Rate 150 Flow rate (l/s)
Pressure

100
250
50
200
150 0

100 −50
27-Oct 30-Oct 2-Nov 5-Nov 8-Nov 11-Nov
Figure 4.24 Wellhead pressure and flow rate during stimu-
lation (waterfrac) of the Detfurth. Positive flow means injec-
tion and negative means production.

To estimate the fracture dimensions, the pressure decline after fracturing is of
special importance. The pressure decreases very slowly but almost linearly. Even
−1
when water was produced at a rate of 10 l s , after the second frac test, the linear
pressure decay continued. Three important conclusions about the frac propagation
can be derived from the slow and linear pressure decline:

3
−1
• The frac behaves like a huge ballon with a storage capacity of about 100 m bar .
The huge storage capacity is indicative for tensile fracturing rather than shearing.
2
• The fracture area is larger than 100 000 m as derived from the storage capacity
and assuming typical mechanical rock properties.
• The hydraulic connection to permeable structures in the far field is poor.
The temperature profile, measured half a year after the frac operations confirms
the assumption of a large fracture (Figure 4.25). The cooled region extended over
a vertical distance of more than 150 m. Adjacent clay and siltstone layers were
obviously penetrated by the fracture showing that the frac propagation is not
confined by these layers as is commonly expected. In particular, a hydraulic link
was formed between the Detfurth-sandstone and the upper Solling-sandstone. This
link forms the basis for testing the concept of a closed circulation between both
layers, separated by a packer in the well.
The hydraulic fracture characteristics at near reservoir pressure have been
investigated in several tests. Figure 4.26 displays the pressure and flow rate for a
cyclic test performed in 2004.
The test consisted of an injection period with cooled water and the succeeding
production of warm water out of the frac after a period of heating-up. As shown,
the pressure close and below reservoir pressure can be matched with an unique
fit. The pressure varies linearly versus the square root of time over a wide range