Stress-dependent permeability measurement using the oscillating pulse technique

Permeability of Indiana Limestone samples undergoing deformation in a triaxial cell along several stress paths such as triaxial compression, hydrostatic compression and uniaxial strain was measured using the oscillating pulse technique. This technique consists of applying a sinusoidal pressure wave at the upstream end of the sample and recording the pressure-time behavior at the downstream end. The solution to the general one-dimensional diffusivity equation can be obtained by applying the appropriate boundary conditions to give a method of evaluating diffusivity and permeability in a relatively short time. The process of sending a pulse to evaluate permeability has been programmed in Visual Basic which can be incorporated into any regular triaxial testing automated routines to measure permeability at different stages of loading until failure. Stored waveforms of the upstream and downstream pressure responses with time are analyzed to evaluate the attenuation and phase shift in the waveform. For comparison purposes, permeability was also measured using steady-state methods for samples subjected to a hydrostatic compression stress path and good correlations were observed. During triaxial compression failure tests, in most tests the permeability decreased continuously until the end of the test. Permeability also decreased with increasing confining pressure, while under uniaxial strain (i.e. one-dimensional compaction (k(0)) tests), there was a sudden reduction of permeability at a particular stress level.