True-Triaxial Testing Apparatus & Double Direct Shear Under Hydrothermal Conditions The true-triaxial apparatus consists of 10,000-PSI pressure vessel (Figure 3) fitted in a servo-controlled biaxial load frame shown above. Measurements of along-fault and across-fault permeability can be carried out in the double-direct shear and single direct shear geometry. The machine is designed for friction, fracture, flow, and deformation experiments under fully-controlled conditions. Each axis of triaxial loading can be applied with force or displacement boundary conditions. Porosity, permeability, and pore fluid volume changes are measured during deformation. Samples can be deformed under drained or undrained loading conditions. Capabilities include double-direct shear and single-direct shear of granular and clay rich simulated fault gouge, natural fault gouge, bare rock surfaces in frictional contact, and in-situ shear fracture of intact samples.

The pressure vessel has removable doors to allow sample access and set-up of electrical and fluid connections (Figure 3). Fluids enter through the platens and access the sample via porous frits (Figure 4). A flexible latex jackets separates pore and confining fluids. Pore-pressure (Pp) and confining pressure (Pc) intensifiers are designed for pressures to 10,000 PSI with a volume of 155 cc. The system is capable of resolving volume changes as small as 1×10-4 cc. Biaxial applied stresses (heavy arrows in Figure 4a) of up to 300 MPa can be applied.

We have also developed a system for measuring acoustic velocity/elastic wave properties and passive acoustic emissions within the biaxial testing machine and double direct shear configuration. The system makes used of pin-type PZT’s (pinducers), used primarily in passive detection mode, and larger (1” diameter) PTZ’s used in both ‘active source’ and ‘passive’ mode. The PZT’s are embedded in specially designed sample grips in the double direct shear configuration (see Figure below). The system is capable of accepting P- and S-mode PZT transducers and can measure one-way travel time through the samples, using established first arrival and waveform correlation techniques (“time-of-flight”), similar to the approach described above for core samples. PZT crystals with low center frequencies are driven by a high voltage (900V) signal generator.

For triaxial experiments, four cells are available for confining pressures up to ~100 MPa and temperatures up to 150 C. The vessels are interchangeable within the load frames, but differ slightly in their construction. All have several ports for fluid or electrical lines to pass into the vessel, and to allow simultaneous measurement of permeability and ultrasonic wavespeed. In our experiments, samples are typically jacketed in latex or viton, with o-ring seals at the top and base of the sample, and silicon oil is used as a confining fluid.

In typical configuration, both the top and base of the sample are plumbed to pressure/volume controllers, to maintain sample pore pressure and to allow permeability testing. A third pressure/volume controller can be used to maintain confining pressure, and to monitor volume changes of the confining fluid related to sample dilation or consolidation. We also measure the volumetric strain using the external LVDT for axial deformation and the radial strain caliper equipped with a submersible LVDT. Any desired stress path within the apparatus maximum pressure and stress capability can be achieved under (1) stress feedback control (controlled mean and differential stress, resulting strain is monitored), (2) strain control (controlled shortening rate and/or rate of volume change, resulting stresses are monitored), or (3) manual control.

Measurements of permeability can be carried out in any of the testing systems, using either flow-through (steady-state), or transient (sinusoidal loading or pulse decay) techniques. Flow (or pressure) for permeability tests is controlled by high-precision flow pumps plumbed to the upstream and downstream ends of the sample. Flow through tests can be conducted in concert with ultrasonic measurements of Vp and Vs along the core axis. This is achieved by using modified end-caps for fluid distribution that include 1/2” diameter 500 kHz lead zirconate titanate (PZT) shear-wave transducers embedded within a “well” in each end cap. PZT crystals cut in a shear orientation also produce a small P-wave at a similar center frequency and can be excited by an arriving P-wave. This allows us to use the shear wave crystals to measure both P and S waves simultaneously. Velocity is measured in direct time-of-flight mode, using a 900-volt negative impulse as a source pulse with a repetition rate of 100 Hz. The recording frequency is 25 MHz, and each saved waveform is made up of 100 stacked waveforms. The system is also equipped for measurement of electrical resistivity in conjunction with permeability tests, using modified insulated end-caps (however, resistivity and velocity measurements cannot be conducted simultaneously in the system as currently configured).

The High Pressure High Temperature Laboratory houses four standard triaxial vessels capable of deviatoric and confining stresses to 100 MPa on 25 mm diameter samples and similar fluid pressures. High precision pumps are capable of applying confining pressures, deviatoric stresses and fluid pressures. Simultaneous deformation and permeability may be completed at temperatures to ~150C and with liquids or gases as the permeants. Permeability measurements are made as steady "Darcy" tests for moderate permeability materials (>mD) and as pulse tests for low permeability materials (< mD). The systems are equipped with a gas chromatograph to measure multi-component sorption, desorption and swelling response of sorbing media such as coals and shales. Some of the systems are housed in environmental chambers that control temperature to within +/- 0.1 C, in order to minimize the effects of thermally-induced volume changes on fluid flow and deformation measurements. Pore volume change is measured to +/- 1 mm3 (1 µl), displacement is measured to +/- 0.5 µm, displacement rate is controlled to a precision of +/- 10-2 µm min–1, and stress is measured/controlled to a precision of +/- < 1 kPa.