A comparison of hydraulic stimulation experiments in the HDR-Soultz project, France
and in the GeneSys-project, Germany will be presented. Similar stimulation concepts
have been applied in both projects but for completely different rock types.
In the well known Soultz-project massive hydraulic fracturing experiments
(waterfracs) have been performed in granite rock. Typically, water volumes of more
than 10,000 m3 have been injected in order to stimulate large fracture areas and to
form hydraulic links between the three wells at a depth of 5,000m.
The stress field in Soultz is characterised by a great difference between the maximum
vertical stress and the minimal horizontal stress component. Therefore, the stress
field favours shearing as the significant stimulation process. The observed high
intensity of microseismic events and the pressure curves during stimulation indicate
shearing too. On the other hand hydraulic tests after stimulation suggest the
existence of high or infinite conductive fractures around the wells. Further, the
wellbore storage is high (in the order of 1m3/bar) even for a low pressure.
Both properties: the large wellbore storage and the infinite conductive fracture can
not easily be explained by shearing of natural pre-existing fractures. A special pull
apart mechanism seems to be likely that keeps the fracture “wide” open in the
vicinity of the wells. Such a mechanism could be the development of a tensile
fracture within a fault zone. Due to the shearing of the boundaries of the fault zone
the tensile fracture inside remains open and causes the observed high wellbore storage.
The GeneSys-Project of BGR/GGA in Hannover is directed to the development of single
well concepts for direct use of geothermal energy. In a test well about 80km NE of
Hannover stimulation experiments in a tight sandstone layer (formation “Bunter”) were
performed. The stress field here is much more isotropic than in Soultz. At the target
depth of about 3,8km the minimal horizontal stress component accounts for
approximately 80% of the overburden. But the minimal horizontal stress changes
significantly between the different layers at similar depth (clay stone, sandstone).
The main stimulation operation was performed by injecting 20,000 m3 of fresh water
with an injection rate up to 50l/s. Almost no seismic events could be recorded from
the surface seismic network during this operation indicating only weak seismic events
(M < 0.5).
Extended pressure oscillations and a very weak pressure decline after Shut in gave
evidence for a huge wellbore storage (≈ 100m3/bar), much higher than ever observed in
Soultz.
All the observations during the stimulation indicate that a huge tensile fracture was
created here. Surprisingly, the conductivity of the tensile fracture was very high
(“infinite”) even for a pressure much lower than the frac extension pressure. This
behaviour is contradictory to the expected one. In general it is assumed that the
hydraulic conductivity of fractures is low if no shearing occurs.
The explanation of the infinite conductive fracture has to take into account the
geological structure of alternating layers with different minimal stress. Likely, the
fracture extension pressure was controlled by the minimal stress not of the sandstone
layer itself but by ambient claystone layers. The fracture extension pressure is much
higher than the minimal stress in the target sandstone leading to an inelastic
deformation inside the sandstone layer.
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