Hydraulic transport through calcite bearing faults with customized
roughness: Effects of normal and shear loading.
Understanding fluid flow in rough fractures is of high importance to
large scale geologic processes and to most anthropogenic geo-energy
activities. Here, we conducted fluid transport experiments on Carrara
marble fractures with a novel customized surface topography.
Transmissivity measurements were conducted under mechanical loading
conditions representative of deep geothermal reservoirs (normal stresses
from 20 to 70 MPa and shear stresses from 0 to 30 MPa). A numerical
procedure simulating normal contact and fluid flow through fractures
with complex geometries was validated towards experiments. Using it, we
isolated the effects of roughness parameters on fracture fluid flow.
Under normal loading, we find that i) the transmissivity decreases with
normal loading and is strongly dependent on fault geometry ii) the
standard deviation of heights (RMS) and macroscopic wavelength of the
surface asperities control fracture transmissivity. Transmissivity
evolution is non-monotonic, with more than 4 orders of magnitude
difference for small variations of macroscopic wavelength and RMS
roughness. Reversible shear loading has little effect on transmissivity,
it can increase or decrease depending on the combined contact geometry
and overall stress state on the fault. Finally, irreversible shear
displacement (up to 1 mm offset) slightly decreases transmissivity
contrary to common thinking. The transmissivity variation with
irreversible shear displacements can be predicted geometrically at low
normal stress only. Finally, irreversible changes in surface roughness
(plasticity and wear) due to shear displacement result in a permanent
decrease of transmissivity when decreasing differential stress. We
discuss the implications for Enhanced Geothermal Systems stimulation.