GEOMECHANICAL TESTING

Elastic properties – Young’s modulus, bulk modulus, shear modulus and Poisson’s ratio can be determined via static measurements under confining pressure. Strain gauges can measure both axial and lateral deformation of the sample. Dynamic elastic properties can also be measured by means of acoustic wave speed methods over a range of pressures. Specimens are subjected to confining pressures in the range 0-400 MPa.  

Unconfined compressive strength – a laterally unrestrained specimen is loaded until failure. 

Confined compression/extension to failure – a specimen is laterally confined by an oil confining medium and loaded until failure in order to determine the Mohr Coloumb failure criterion. A number of tests on core plugs is required to characterise behaviour. Degree of water saturation affects strength. Tests are continued post-failure to determine frictional sliding resistance on the fault plane produced.

An example of a shale sample that has been loaded in compression to failure. The orientation of the sample is parallel to foliation.

Brazilian disc test – this is an indirect method to obtain the tensile strength of a brittle material. A thin circular disc is compressed along its diameter until failure.

Direct shear measurements – low pressure friction measurements are made on an ELE Direct Shear apparatus. Higher pressure measurements can be made in the triaxial apparatus for sliding on an inclined sawcut plane.

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