Failure mechanical and acoustic behavior of brine saturated-sandstone containing two pre-existing flaws under different confining pressures

Abstract

To better understand the mechanical, acoustic and failure behaviors of sandstone in deep saline aquifers, conventional triaxial compression tests were carried out on sandstone specimens containing two pre-existing three-dimensional (3D) flaws. First, pre-flawed sandstone specimens were saturated under NaCl solution with a salinity of 20 wt%. Then both dry and brine-saturated sandstone specimens were loaded under triaxial compression. As the flaw angle increased, the peak strength and crack damage stress of the dry and saturated pre-flawed sandstone specimens increased. The peak strength and crack damage stress of dry specimens increased linearly, while those of brine-saturated specimens increased nonlinearly with confining pressure. During triaxial loading, P-wave and AE signals were monitored in real-time for the sandstone specimens. The evolution of P-wave velocity and AE events can be characterized as having five stages based on the internal damage of sandstone specimens. In general, the accumulated AE count can be summarized as follows: very few or no AE counts, increases linearly, increases exponentially and increases stably. The P-wave velocity was also observed as: increases rapidly, increases stably, decreases unstably and keeps constant. Finally, by using X-ray micro CT observations, two crack coalescence modes between the two pre-existing flaws were identified, i.e., shear crack coalescence in the ligament region and indirect coalescence outside the ligament region. The second coalescence mode was only observed in the sandstone specimen with a flaw angle of 45° under low confining pressures. The internal opaque cracks were clearly displayed in the reconstruction images as a curved surface.