Lab Services

Unconfined Compression Testing


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.


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