Microcracking during triaxial deformation of porous rocks monitored by changes in rock physical properties, II. Pore volumometry and acoustic emission measurements on water-saturated rocks

Abstract

We report results from a series of laboratory triaxial-deformation experiments performed on samples of Darley Dale sandstone at servo-controlled constant strain rate and constant pore-fluid pressure. During deformation, the volume of water either expelled from the samples during compaction or injected into the samples during dilatancy in order to maintain a constant pore-fluid pressure was continuously monitored throughout each experiment. In addition, the parameters of differential axial stress, axial strain, and acoustic emission (AE) characteristics were also recorded. Complete AE waveforms were also captured and stored by means of a transient recorder, and Fourier analysis of these waveforms was subsequently carried out. The reported experiments were all performed in a conventional, high-pressure, gas-medium triaxial cell, but with the pore-fluid pressure maintained constant using a new servo-controlled fluid pressure intensifier and pore volumometer. The AE and pore volumometry measurements on water-saturated rocks reported in this paper are complementary to the measurements of elastic-wave propagation parameters made on dry rocks and reported in a companion paper in this issue. The two suites of data have been integrated to infer relations between crack density parameters calculated from elastic-wave velocity measurements and the directly measured pore volume during deformation. Our results show a distinct, positive correlation between changes in the axial crack density parameter ( ϵ X ) and changes in the pore volume during deformation once fluid expulsion due to elastic pore collapse has been corrected for. These two parameters have then been combined to obtain an estimate of changes in the mean aspect ratio of propagating dilatant cracks. The results suggest that, under moderate confining pressures, axially aligned dilatant cracks are likely to bow open elastically prior to significant crack extension and growth.