Mechanical and hydraulic transport properties of transverse-isotropic
Gneiss deformed under deep reservoir stress and pressure conditions.
Abstract
In central Europe, many geo-energy reservoirs have revealed to be hosted
in transverse isotropic crystalline rock, where the rock’s mechanical
and hydraulic transport properties are poorly constrained. Here, we
performed triaxial experiments on Cresciano Gneiss samples under stress
(25-40 MPa) and fluid pressure (5 MPa) conditions. We tested 5 different
foliation orientations towards the major principal stress (0, 30, 45,
60, 90). During deformation, we measured the porosity evolution and
acoustic emission activity of the samples. In addition, we measured the
axial permeability and P-wave velocity of the samples both during
isostatic confinement and after sample failure. Our results show that
the mechanical and hydraulic transport properties of transverse
isotropic tight crystalline rocks can be separated into two classes.
First, the mechanical properties such as onset of dilatancy, yield
stress, peak strength and residual strength, follow a ”U-type”
anisotropy towards foliation angle, with maximum values at 0 and 90 and
minima between 30 and 45. These properties, as well as the porosity
variation during deformation which follows an inversed ”U-type” shape
can be explained by anisotropic wing crack models. Second, the
volumetric physical properties (permeability and P-wave velocity) follow
a ”decreasing order” shape towards foliation angle, with maximum
values at 0 decreasing to the minimum at 90. These properties show a
high dependence on the stress state and the wave path. We discuss the
implications of these results for deep geothermal energy prospection,
and for reservoir stimulation and operation.