PETROPHYSICAL STUDIES OF SANDSTONES UNDER TRUE-TRIAXIAL STRESS CONDITIONS

Development of an innovative polyaxial (true-triaxial) stress loading system is described. The system was originally designed to determine ultrasonic velocities, fluid permeability and elastic properties on cubic rock specimens in which sets of orientated fractures and microcracks had been introduced. The original system was used also in conjunction with differential strain analysis (DSA) to compare DSA with ultrasonic shear-wave splitting (USWS) for predicting the in situ state of stress in rock masses. The system was then modified to incorporate acoustic emission (AE) sensors, in addition to the ultrasonic velocity transducers, to investigate laboratory-induced fracturing of a sandstone and to analyse directly the mechanics driving the fracturing. The loading system has subsequently been modified further to incorporate a pressuresealing scheme, to enable high pore pressures to be achieved, and a dedicated loading frame with all principal stresses servo-controlled. In this latter form the system has been used to study directional permeability and electrical conductivity, pore volume change and capillary pressure characteristics at elevated external stresses and pore pressures, in addition to the measurements for which it was originally designed.