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Understanding the Geological Basis of the Iowa Pore Index

Abstract

Petrophysical analysis of reservoir rocks commonly employs helium and mercury porosimetry on 25- to 100-mm (one- to four-inch) diameter core samples to characterize the pore network of reservoir rocks. For approximately thirty years, the Iowa Pore Index (IPI) method has been used to analyze the pore systems of crushed carbonates for road construction. This technique, when combined with x-ray diffraction and x-ray fluorescence (to measure dolomite content and clay content, respectively), has been shown to be effective in predicting the performance of aggregates in Portland cement concrete. The IPI method has the potential to add value inexpensively during core analysis by (1) providing a macropore-to-micropore ratio, (2) analyzing a large volume of rock (4.5 kilograms, approximately equal to a foot-long, four-inch diameter limestone core with 25% porosity), (3) leaving the sample unpolluted (unlike mercury intrusion), (4) being relatively quick and inexpensive (the test takes less than 30 minutes and the only consumable is water). This research aims to understand the geological factors – i.e. depositional environment, facies, grain type, pore type, texture, and paragenesis – that govern the results of the IPI method. To accomplish this, the IPI method has been run on a spectrum of Iowa carbonate rocks (both limestones and dolostones) which were subsequently analyzed quantitatively via helium and mercury intrusion porosimetry, and qualitatively via thin section petrography and scanning electron microscopy. In this way, we investigate the applicability of this method to petroleum industry problems by comparing the IPI results to those obtained using standard petroleum industry reservoir characterization procedures.