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

Small angle neutron scattering (SANS), ultra-small angle neutron scattering (USANS), and backscatter electron (BSE) imaging, along with neutron computed tomography (NCT) were used in this study to experimentally quantify pore size, distribution, and connectivity of dissolved limestone geometries. Eight samples of Indiana limestone of two different initial permeabilities (2-4 mD and 70 mD ) [millidarcy] were reacted with HCl [hydrochloric acid] solutions at differing pH (2 and 4), and flow rates (0.1 and 10 cm³/min) [cubic centimeters per minute] to describe a broad range of parameters that affect limestone dissolution. NCT was first used to image the dissolution structure, and each core was then cut into nine sample sections at various distances from the inlet face to the outlet of dissolution to the outlet. These were used for (U)SANS and SEM/BSE analysis to characterize changes in the pore structure throughout the entire core. The scattering curves obtained from (U)SANS were combined with autocorrelation analysis of the BSE images to characterize porosity over a wide range in length scales from approximately 1 mm [millimeter] to 1 nm [nanometer].

Preliminary macro-observations and neutron tomography show preferential flow paths that form through the limestone at the lowest pH with relatively no change in porosity at the highest pH. The permeability of each sample controls the penetration and degree of branching of each flow path into the cores. The scattering curves obtained from SANS-USANS are combined with the data from the BSE images to give a wide range in length scale from 1 mm [millimeter] to 1 nm [nanometer]. Scattering data show as initial permeability and pH increase the volume of nano-pores within the limestone decreases and the volume of micro-pores increases.