Velocity inversion for changes in crack aspect ratio spectra during heat treatment of Berea sandstone

Abstract

Summary This paper presents an analysis of a heat treatment study conducted on Berea sandstone samples that employs the theoretical Cheng-Toksoz inversion model for the porecrack aspect ratio spectra. A modified velocity inversion using a differential effective medium (DEM) theory is developed that yields a spectrum of pore-crack aspect ratios. These spectra are derived from a joint inversion of dry P- and S-velocities pressure dependencies. The backcalculated and measured velocities match each other very well with an average error of less than 5%. The model is applied to a suite of twelve Berea sandstone samples which have been subjected to treatment temperatures of 300°C and 600°C. The velocities show little changes after samples are heat treated to 300°C but drop significantly, about 10%, after heat treated to 600°C. The velocity inversion model predicts an increase in concentration of small aspect ratio cracks as a rock is heat treated to progressively higher temperatures. Examples showing changes in porosity, density, NMR spectra, mercury injection spectra and thin section analysis strongly support the creation of thin cracks as samples are heat-treated to high temperatures. Independent documentation of thin crack formation due to thermal treatment is consistent with the velocity inversions, lending credence to the use of velocity inversions for microstructural characterization.