Pressure-induced microcracking and grain crushing in berea and boise sandstones: acoustic emission and quantitative microscopy measurements

Abstract

The micromechanics of hydrostatic compaction and porosity reduction was investigated by acoustic emission (AE) and microscopy measurements. The room temperature hydrostatic tests were conducted at confining pressures up to 550 MPa and constant pore pressures up to 10 MPa. Contrary to our expectation, the porosity reduction processes were very efficient in generating AE. The hydrostat of a porous sandstone usually has an inflection point which separates the compaction process into two stages. Distinctly different AE activity, AE efficiency and microstructure are associated with each of these two stages of compaction, implying a transition in micromechanism at the inflection point. Probably the dominant micromechanical processes for porosity reduction was grain slip and rotation in the first stage of compaction and grain crushing in the second stage. Stress-induced cracking associated with the grain crushing introduces relatively thin, intragranular cracks. At the same time, the grain crushing provides additional degrees of freedom for grain rotation allowing some of the crushed grains to move into the pore space. Both mechanisms modify the pore dimension as well as aspect ratio. We developed a stereological technique to characterize quantitatively the pore dimension distribution. We also used an indirect technique based on Waish's (1965) theoretical analysis to infer microcrack porosity. Grain crushing eliminated the relatively large pores (of dimension ranging from 200 μm to 700 μm) in a compacted Boise sandstone sample. Even though the overall porosity decreased with pressure, the microcrack porosity actually increased with compaction. Most of the pressure-induced microcracks were of relatively high aspect ratios. The Kaiser effect and compaction creep were also investigated by cyclic tests and time-dependent AE measurements.