Influences of pre-structuration of a reservoir on permeability evolution in fracture zones. Insights from mechanical testing on an alteration profile

In the international energy transition, geothermal
energy plays a role as a renewable energy which almost
does not emit CO2 and allows the reduction of
dependency on fossil fuels. Geothermal energy in a
crystalline basement is part of the different deep
geothermal sources which can be used for heating or
electricity production. In 2018, the heating generated
from deep geothermal plants was up to 2.6 Twh in
France and is planned to increase. For the upcoming
2030, the objective in France is to have 38% of
renewable energy in the final heat consumption, where
geothermal energy has its place (French energy
transition law for green growth n°2015-992).
Geothermal production in crystalline basement relies
on the presence of fractures and sometimes stimulation
techniques. In France, such exploited sites are located
in Alsace (Upper Rhine Graben, URG) where a hot
brine is harvested in the granitic basement to produce
electricity (Soultz-sous-Forêts) or heat (Rittershoffen).
However, recent unexpected seismic events occurred
during the stimulation of new sites and lead to stop deep
geothermal exploration in Alsace.
For the resource development of geothermal reservoirs,
the understanding of the properties of fractures and
fracture networks has to be investigated.
Uniaxial compression strength, triaxial compression
strength and brazilian disk tests measurements were
performed on samples collected along a fault zone from
Schauinsland mine in the Black Forest. These samples
are considered as analogues of the deep geothermal
reservoir of Soultz-sous-Forêts in the URG. A decrease
of the tensile strength and elastic modulus was
measured along the fault zone while the cohesion value
is much higher than expected in the core zone. The
Poisson ratio is the highest in the damage zone which
indicates that the samples are heterogeneous and need
to be investigated at a smaller scale.