Fracture aperture maps used to study reactive transport, channelization, and permeability evolution in carbonate rocks.

Fractures in geological formations may enable migration of environmentally
relevant fluids, as in leakage of CO2 through caprocks in geologic carbon
sequestration. We investigated geochemically induced alterations of
fracture geometry in Indiana Limestone specimens. Experiments were the
first of their kind, with periodic high-resolution imaging using X-ray
computed tomography (xCT) scanning while maintaining high pore pressure
(100 bar). We studied two CO2-acidified brines having the same pH (3.3)
and comparable thermodynamic disequilibrium but different equilibrated
pressures of CO2 (PCO2 values of 12 and 77 bar). High-PCO2 brine has a
faster calcite dissolution kinetic rate because of the accelerating effect
of carbonic acid. Contrary to expectations, dissolution extents were
comparable in the two experiments. However, progressive xCT images
revealed extensive channelization for high PCO2, explained by strong
positive feedback between ongoing flow and reaction. The pronounced
channel increasingly directed flow to a small region of the fracture,
which explains why the overall dissolution was lower than expected.
Despite this, flow simulations revealed large increases in permeability in
the high-PCO2 experiment. This study shows that the permeability evolution
of dissolving fractures will be larger for faster-reacting fluids. The
overall mechanism is not because more rock dissolves, as would be commonly
assumed, but because of accelerated fracture channelization. Fractured
cores of Indiana Limestone were scanned with x-ray computed tomography
(xCT) and the fracture geometry was analyzed as described in Deng et al.
(2016). These fractured cores were used in experiments with CO2-acidified
fluids and the evolution of fracture geometry and permeability were
measured, as described in Deng et al. (2015). The 3D reconstructed images
were used to generate aperature maps and statistical representations,
which were used as the initial conditions for reactive transport models in
Deng and Peters (WRR, submitted Dec 2017). The reactive transport
simulations studied the effects of mineral heterogeneity using calcite
mineral maps presented in Ellis and Peters (2016). ... Download the
README.txt file for additional details about the dataset.