Collapse of reacted fracture surface decreases permeability and frictional strength

Geochemical and geomechanical perturbations of the subsurface caused by
the injection of fluids present the risk of leakage and seismicity. This
study investigated how flow of acidic fluids affects hydraulic and
frictional properties of fractures using experiments with 3.8 cm-long
specimens of Eagle Ford shale, a laminated shale with carbonate-rich
strata. In low-pressure flow cells, one set of samples was exposed to an
acidic brine and another set was exposed to a neutral brine. X-ray
computed tomography and x-ray fluorescence analysis revealed that samples
exposed to the acidic brine were calcite-depleted and had developed a
porous altered layer, while the other set showed little evidence of
alteration. After reaction, samples were compacted and sheared in a
triaxial cell that supplied normal stress and differential pore pressure
at prescribed sliding velocities, independently measuring friction and
permeability. During the initial compaction, the porous altered layer
collapsed into fine particles that filled the fracture aperture. This
effectively impeded flow and sealed the fracture, resulting in a decrease
in fracture permeability by 1 to 2 orders of magnitude relative to the
compressed unaltered fractures. During shear, the collapsed layer of
fine-grained particles prevented the formation of interlocking
micro-asperities resulting in lower frictional strength. With regard to
subsurface risks, this study showcases how coupled geochemical and
geomechanical processes could favorably seal fractures to inhibit leakage,
but also could increase the likelihood of induced seismicity. These
findings have important implications for geological carbon sequestration,
pressurized fluid energy storage, geothermal energy, and other subsurface
technologies. Five 3D X-ray computed tomography images of Eagle Ford shale
rock coupon pairs, one for each condition shown in Figure 3 in the related
publication. Details of the data in this part are presented in README.txt.
For more information about the experiments and methodology, please refer
to related publication.