AE from undrained and unjacketed tests on sandstone

Saturated specimens of Berea sandstone were subjected to confining pressure p and compressed axially under a plane strain condition; the biaxial deformation state is convenient for measuring axial and lateral displacements. Undrained testing involved development of pore water pressure u during application of deviatoric stress, and unjacketed tests maintained the condition of u = p. In addition, the specimens were instrumented with eight acoustic emission (AE) sensors for monitoring the microseismic activity during the tests. AE events were recorded and post-processed for rate and location analyses. A significant difference was observed between unjacketed and undrained compression in terms of AE rate change with loading. For an unjacketed test, where the pore pressure inside the rock was equilibrated with the confining pressure, the number of AE events per load step remarkably increased when the axial stress reached approximately 70% of the peak, similar to the behavior of dry specimens. However, in an undrained test, where the fluid was not allowed to leave the specimen, only a few microseismic events were recorded prior to failure of the specimen. The abrupt change in the slope of the AE rate happened only when the pore pressure in the rock decreased. This effect could be explained by the delayed tendency of the rock to dilate under an undrained condition.