Triaxial compression testing is performed to:
- provide parameters for hydraulic fracturing design
- provide data points for determining a failure locus (i.e. Mohr envelope)
- As a component in wellbore stability, sand production and subsidence calculations
In a conventional triaxial compression test, a cylindrical core sample is loaded axially to failure, at constant confining pressure. Conceptually, the peak value of the axial stress is taken as the confined compressive strength of the sample. In addition to axial stress, axial and radial strains may be monitored during this test, to determine basic elastic constants (Young’s Modulus, E, and Poisson’s ratio).
If triaxial testing is performed at several confining pressures and coupled with unconfined compression and tensile test data, a representative failure envelope can be constructed. The selected confining pressures for triaxial testing are generally spread over a range from very low to beyond the maximum anticipated in-situ effective stress conditions. Measurements can be performed at in-situ temperature and pore pressure.
Testing Equipment and Setup
The triaxial compression system, available at NSI Laboratories, is used to perform this type of testing. An axial load is applied with a servo-controlled actuator. Confining pressure and pore pressure are hydraulically generated. Axial force up to 50,000 lbf can be applied to the samples up to one and one half inches (1.5”) in diameter. Axial stress is monitored with a load cell. Confining pressure and pore pressure are monitored with conventional pressure transducers. Axial and radial strains are measured using strain extensometers. Tests can be conducted at temperatures up to 400° F.
- A cylindrical sample, with a length-to-diameter ratio of two (common diameters are one to one and one half inches) is cut with an inert fluid and endground flat and parallel, in accordance to ISRM standards (recommended tolerance in end parallelism is ± 0.001 inches).
- The sample can be pre-saturated with an appropriate native fluid (or other desired fluids) or the tests can be conducted “dry”.
- The sample is installed between hardened steel endcaps and this assembly is sealed with a thin, deformable, heat shrink jacketing material. The jacket prevents confining fluid from penetrating into the sample and allows independent control and monitoring of the confining and pore pressures during testing. The endcaps are ported to allow application of pore pressure and/or flow if permeability is measured.
- If strain measurements are being performed, axial and radial strain measurement devices are mounted on the sample fixture.
The procedures for conducting a triaxial compression test are, for the most part, relatively standardized. The assembled sample and instrumentation fixtures are installed in a pressure vessel. After this, typical procedures might include the following steps:
- Fill the pressure vessel with hydraulic confining fluid. Raise the confining pressure to a nominal value (100 psi) at a servo-controlled rate (1 psi/s). This initial confining pressure is applied so that there will always be at least a small difference between confining pressure acting outside of the jacket and pore pressure in the rock (inside the jacket). Otherwise leakage will occur.
- Often, freezing is used to maintain the integrity of unconsolidated samples during preparation. Obviously, this is not desired; however, it is sometimes the only feasible method of preparing such samples. For frozen samples, a thawing period is allowed. While thawing, strains are allowed to equilibrate.
- The confining pressure and the pore pressure are simultaneously increased at a controlled rate (1 psi/s) until the pore pressure reaches a target value.
- The pore pressure is maintained constant and the confining pressure is increased, at a controlled rate, until a pre specified value is reached.
- The axial stress difference is increased at a rate corresponding to an axial strain rate of 10-5/s. Alternatively, rather than controlling the axial strain rate, the axial stress rate can be controlled. Loading is continued until the sample fails.
- The sample is unloaded slowly, the pressure vessel is emptied and the sample assembly is disassembled.
- The sample is examined, documented and archived in a specified manner.
Test Results, Calculations, and Reporting
Experimental results are represented as stress-strain curves, and tabulated values of elastic constants and strength. The stress-strain data are used in determining the compressive strength and elastic constants. In brittle or elastic-perfectly plastic or strain softening materials, confined compressive strength at the confining pressure used in a triaxial test is taken as the maximum effective axial stress (total axial stress minus a percentage of the pore pressure) accommodated by the sample. When strain hardening occurs, other criteria are adopted for selecting the "strength." Elastic constants are determined over linear sections of the stress-strain curves.