UNDERSTANDING HOW CARBONATE MICROPORES AFFECT THE IOWA PORE INDEX METHOD FOR EVALUATING HIGHWAY AGGREGATE

The Iowa Pore Index test is a method employed by several Midwestern State Departments of Transportation to determine the volume ratio of macropores-to-micropores in a rock aggregate by means of water intrusion. Results of the Iowa Pore Index test have been used in conjunction with x-ray diffraction (to measure dolomite content) and x-ray fluorescence (to measure clay content) analyses to evaluate the susceptibility of carbonate rocks to premature deterioration when used in road construction. In this test, 4.5 kilograms of oven-dried crushed carbonate is intruded with water progressively at 240 kilopascals. Readings of intruded volume are taken at 1 and 14 minutes corresponding to macropore and micropore volumes, respectively. The Iowa Pore Index test is interesting more broadly because it is fast, non-destructive, inexpensive, and environmentally friendly, hence it has the potential to replace mercury porosimetry and be integrated in any petrophysical lab.

This research aims to understand the geological factors (depositional environment; facies; grain and pore types; texture; and paragenesis) responsible for the results of the Iowa Pore Index test. End-member samples of various geologic ages are collected around Iowa to represent different combinations of good and bad porosity and clay content. The pore index of each sample is calibrated quantitatively via helium and mercury porosimetry and qualitatively via thin section.

Thin section combined with mercury intrusion porosimetry allows characterization of cm-to nm-scale pore networks. Even the most homogeneous sources have at least three different rock types. Total Iowa Pore Index (macropore plus micropore volume) is linearly correlated to helium porosity weighted by rock-type abundance. Limestones pore-throat size distributions tend to include smaller pore throat sizes than dolostones. Samples with high secondary IPI generally have higher porosity at smaller pore-throat size distributions. Future work involves integrating XRF chemistry analysis and characterizing the nature of μm-to nm-scale porosity using SEM.