Reports: ND852265-ND8: Geophysical Monitoring of Mechanical and Chemical Alteration of Frictional Discontinuities
Antonio Bobet, Purdue University
Chemical alteration of rock joints has a large effect on hydraulic
conductivity and on flow paths of joints and joint sets. The goal of the
research is to investigate seismically the changes in fracture specific
stiffness associated with chemo-mechanical alteration of fractures. This is
achieved by performing laboratory-scale experiments on a fracture on Indiana
limestone under constant uniaxial compression, while chemical and physical
alterations of the joint are induced by permeating a reactive fluid (white
distilled vinegar) through the fracture. Full waveform measurements of
compressional and shear waves are taken from nine pairs of wave transducers
installed on each side of the fracture to monitor any changes, in real time,
that occur during the reactive flow. Figure 1 shows the position of the
transducers across the fracture.A set of experiments was conducted on fractured Indiana
limestone. The preparation and testing procedures were carefully duplicated to
ensure repeatability of the tests. First, a fracture was created on a cubic
specimen of Indiana limestone by diametrical compression, similar to the
Brazilian test, by applying a load to two opposite sides of the sample using
steel rods. Transducers, embedded into the load platens, were attached to the
sides parallel to the fracture using honey as a couplant. A constant uniaxial
compression stress of 4 MPa was applied during 200 minutes to stabilize the
contact between the honey and the rock. After that, the stress was maintained
for four hours. Once this was done, water was permeated through the fracture
for five days under a constant pressure head of 100 kPa. At the end of this
period, vinegar was permeated under the same constant pressure head for six
days. During the entire duration of the test, transmitted and reflected
compressional and shear waves were taken together with periodic measurements of
flow rates and pH of the fluid exiting the fracture. This allowed us to relate
changes of seismic response with flow and pH. Prior to and after each
experiment, the roughness of the fracture was obtained using a laser
profilometer.A MATLAB code was written to calculate the specific
stiffness of the fracture for normal incident waves. The code computes the
ratio of reflection to transmission coefficients using the amplitudes within
the range of the dominant frequencies obtained from a Fast Fourier Transform of
the P- and S-waves.Figure 2 shows the normal (left) and shear (right) specific
stiffness of a representative fracture during the test. The colors in the
figure represent the different transducers distributed along the fracture
plane. As one can see in the figure, the normal and shear specific stiffness
dropped as water entered the fracture. In fact, the change of peak-to-peak
amplitude of the normal and cross-transmitted incident compression and shear
waves was used to detect the water front arrival time. The results demonstrate
(not shown in the figure) that the arrival time of the fluid front at specific
locations of the fracture was inversely proportional to the specific stiffness.
In other words, fluid invaded first areas with relatively low specific
stiffness and then propagated to areas with higher specific stiffness. Figure 2
also shows that water flow increased the specific stiffness of the fracture
over time. At about 6,000 minutes after the start of the test, an abrupt drop
in specific stiffness is observed, which corresponds to the initiation of the
vinegar flow. As observed with water, the specific stiffness increased with
time under all transducers, but after about three days, it stabilized under the
majority of the transducers. At this time, the flow rate and pH became
constant. The interpretation of these data is underway and additional tests are
still necessary to understand better the interplay that exists between
chemical, mechanical and seismic phenomena.Figure 1. Transducer
LayoutFigure 2. Fracture
Specific Stiffness with Time. P- (left) and S- (right) Wave Transducers