Page 240 - Geothermal Energy Systems Exploration, Development, and Utilization

216 4 Enhancing Geothermal Reservoirs
The treatment started with an injection test with flow rates between 0.3 and
3
3
−1
0.57 m min . In total 250 m were injected into the reservoir at a maximum
wellhead pressure of 416 bars.
Subsequently, a leakoff test was carried out to obtain the frac gradient
−1
(0.16 bar m ). Then, a step-rate test followed to calculate the friction and tortuosity
at the perforation. Finally, the gel-proppant treatment was performed where 95
3
tons of proppants and 280 m of cross-linked gel were injected into the Lower
3
−1
Dethlingen formation with a flow rate of 4 m min .
The second gel-proppant treatment was carried out from 23 August to 24 August
2007 in the sandstones of the Upper Dethlingen. The treatment had a similar
design as the previous one. The bridge plug was set in 4123 m depth and the
interval above from 4118 to 4122 m was perforated. The treatment started with an
3
injection test with rates between 0.3 and 0.62 m min −1 and a total volume of
3
170 m . The leakoff test analysis yielded a frac gradient of 0.15 bar m −1 .In the
3
following stimulation treatment, 113 tons of proppants and 310 m of cross-linked
−1
3
gel were injected at flow rates between 3 and 3.5 m min .
After stimulation of the reservoir sections a CLT with a nitrogen lift in conjunction
with a flowmeter profiling was performed to test the stimulated intervals. In
advance, additional perforations (deep penetration charges) were carried out in the
3
sandstone sections. Over a period of 12 hours approximately 300 m were produced.
During production two runs (up and down respectively) were performed by the
flowmeter to obtain the inflow profile. The results of the flow profile show that 30%
of flow originates from the volcanic rocks. Nearly 50% of flow can be attributed
to the first gel-proppant treatment and 15% is due to the second gel-proppant
treatment. Only 5% can be assigned to the post perforations. A possible reason
might be the drilling fluid, which was used to build a filter cake at the borehole
wall to protect the reservoir. To enhance the performance of the post perforations
it is intended to acidize these intervals.

4.10.1.8 Conclusions
Development of a technology to stimulate deep geothermal reservoirs in sedimen-
tary basins was the purpose of installing the downhole geothermal laboratory in
Groß Sch¨ onebeck. The results reflect the learning curve from several reservoir
hydraulic fracturing treatments in both wells. These experiments are major steps
toward developing a procedure to increase the thermal water productivity from a
prior low permeable sedimentary reservoir. The obtained values of productivity
seem to show the feasibility of geothermal power production from a sedimentary
geothermal environment.
From these experiences we give the following recommendations for other
potential drill sites within the same environment:

1) In general, the stimulation treatments should be specially designed in relation
to the different characteristics and properties of the reservoir rocks. This
implies that several treatments in a well, which have to be isolated in advance
aretobeperformed.