Introduction
Unconfined compression testing is performed to:
- provide a wellbore stability indicator
- provide data points for determining a failure envelope
- aid in wellbore stability, sand production and subsidence calculations
- aid in coalbed methane cavitation design
In an unconfined compression test, a cylindrical core
sample is loaded axially to failure, with no confinement (lateral
support). Conceptually, the peak value of the axial stress is taken as
the unconfined compressive strength of the sample. In addition to axial
stress, axial and radial strains may be monitored during this test, to
determine elastic constants (Young's Modulus, E, and Poisson's
ratio).
Testing Equipment and Setup
This type of testing is conducted on one of several load
frames available at NSI Laboratories. An axial load is applied with a
servo-controlled hydraulic actuator. Available actuators can deliver up
to 50,000 lbf. Axial stress is monitored with a load cell. Axial and
radial strains are measured using extensometers for measuring
displacements. Tests can be conducted at representative temperatures up
to 400° F.
Sample Preparation and Testing
- A cylindrical sample, with a length-to-diameter
ratio of two (common diameters are one to one and a 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.
- If strain measurements are being performed, axial
and radial strain measurement devices are mounted on the
sample.
- Axial stress is applied to the sample. The axial
stress is applied either at a controlled stress rate or a controlled
axial deformation rate. Loading is continued until the sample
fails.
- The failed 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.