When unconventional resource plug samples can be tested for permeability, the pulse decay method provides a rapid,
accurate result for very low permeability cores (down to approximately 1 nd). Measurements are made at representative
confining stress up to 10,000 psi. Equipment designs are available for either gas or liquid flow and follow the
improvements indicated by Jones in SPE 28450. Shale/mudrocks are often tested at as-received water saturation
resulting in effective kg or effective ko. The method represents the key part of the combined-method, stationary-liquid,
approach for relative permeability in tight gas sands (Dacy, SPE 135427). Generally the pulse decay method requires
a fraction of the time needed by the steady-state plug method.
Pulse decay equipment design facilitates a rapid permeability determination.
At high mean pore pressure a small pressure drop is established and monitored providing permeability as the pressure
pulse decays. For demonstration, the data set in the figure shows near complete depletion of the pulse approaching
5000 seconds. Note, however, that computed permeability was stable by 1000 seconds.
The pulse decay permeability method represents the key part of the combined-method, stationary-liquid, approach
for relative permeability in tight gas sands. Effective kg (md) values annotate the data points between the end
points in the graph above indicating measured kg at 70% Sw of 6 nanodarcies and at 19% Sw of 353 nanodarcies.