During hydraulic fracturing, the oxic hydraulic fracturing
fluid
physically and chemically alters the fracture surface and creates
a “reaction-altered zone”. Recent work has shown that
most of the physicochemical changes occur on the shale fracture surface,
and the depth of reaction penetration is small over the course of
shut-in time. In this work, we investigate the physicochemical evolution
of a calcite-rich fracture surface during acidized brine injection
in the presence of applied compressive stress. A calcite-rich Wolfcamp
shale sample is selected, and a smooth fracture is generated. An acidized
equilibrated brine is then injected for 16 h, and the pressure change
is measured. A series of experimental measurements are done before
and after the flood to note the change in physicochemical properties
of the fracture. High resolution computed tomography scanning is conducted
to observe the fracture aperture growth, which shows an increase of
∼8.3 μm during the course of injection. The fracture
topography, observed using a surface roughness analyzer, is shown
to be smoother after the injection. The calcite dissolution signature,
i.e., surface stripping of calcite, is observed by X-ray fluorescence,
and mass spectrometry of the timer-series of the effluent also points
in the same direction. We conclude that mineral dissolution is the
primary mechanism through which the fracture aperture is growing.
The weakening of the fracture surface, along with the applied compressive
stresses, promotes erosion of the surface generating fines which reduce
the fracture conductivity during the course of injection. In this
work, we also highlight the importance of rock mineralogy on the fracture
evolution mechanism and determine the thickness of the “reaction
altered” zone.