Experimental Limestone Dissolution and Changes in Multiscale Structure Using Small- and Ultrasmall-Angle Neutron Scattering

Small-angle neutron scattering (SANS), ultrasmall-angle neutron scattering (USANS), backscatter electron (BSE) imaging, and neutron computed tomography (NCT) were applied to the study of the pore size, pore distribution, and pore connectivity developed during the experimental dissolution of limestone. Eight cores of Indiana limestone having initial permeabilities of 2-4 and 70 mD were reacted with HCl solutions having a pH of 2 or 4 at flow rates of 0.1 or 10 cm³/min. NCT was used to image the structures developed during dissolution. Nine cross sections of each core from the inlet to the outlet were analyzed with SANS and USANS and with BSE imaging to characterize changes in the pore structure throughout the length of the core after reaction. The scattering curves obtained from SANS and USANS were combined with autocorrelation analysis of the BSE images to characterize porosity over length scales from ∼5 mm to 1 nm. Surface-fractal dimensions were ∼2.3 in the nanopore region, and mass-fractal dimensions were ∼2.75 in the micropore region. The transition from surface- to mass-dominated fractal geometry is at a pore size of ∼100 nm. There was no change in fractal behavior with dissolution, pH, permeability, or flow rate. Porosity was generally greater at the inlet, where most of the dissolution occurred, than at the outlet, where there was little or no reaction. There was also some evidence for porosity reduction near the inlet. The distribution of pore sizes peaked in terms of pore numbers in the nano, micro, and meso range, but there was little change in that distribution with dissolution. There was also little change in porosity in samples that developed preferential flow paths (wormholes), which formed in solutions of low pH and, in particular, at high flow rate. The initial permeability of each sample controlled the penetration and degree of branching of each wormhole into the cores. Samples with wormholes had little additional reaction. The composition of the solutions having a starting pH of 4.0 approached the equilibrium value of 9.5 at the outlet, with little regard to flow rate or permeability. In our experiments, the formation of wormholes and the change in porosity were most strongly influenced by the pH of the infiltrating solution, followed by the flow rate and initial permeability.