Dynamic Three-Dimensional Pore-Scale Imaging of Reaction in a Carbonate at Reservoir Conditions

Quantifying
CO2 transport and average effective reaction
rates in the subsurface is essential to assess the risks associated
with underground carbon capture and storage. We use X-ray microtomography
to investigate dynamic pore structure evolution in situ at temperatures
and pressures representative of underground reservoirs and aquifers.
A 4 mm diameter Ketton carbonate core is injected with CO2-saturated brine at 50 °C and 10 MPa while tomographic images
are taken at 15 min intervals with a 3.8 μm spatial resolution
over a period of 21/2 h. An approximate doubling
of porosity with only a 3.6% increase in surface area to volume ratio
is measured from the images. Pore-scale direct simulation and network
modeling on the images quantify an order of magnitude increase in
permeability and an appreciable alteration of the velocity field.
We study the uniform reaction regime, with dissolution throughout
the core. However, at the pore scale, we see variations in the degree
of dissolution with an overall reaction rate which is approximately
14 times lower than estimated from batch measurements. This work implies
that in heterogeneous rocks, pore-scale transport of reactants limits
dissolution and can reduce the average effective reaction rate by
an order of magnitude.