This contribution presents an early-time solution for permeability
evaluation in pulse-decay tests. A nonlinear governing equation for gas
transport in the sample is derived with consideration of the pressure
dependence of gas compressibility and slippage effect, and the
early-time solution is obtained through the integral balance analysis.
The permeability coefficient can be determined by the proposed solution
through the pressure transients within the early-time stage of the
tests, i.e. before the upstream pressure pulse penetrates through the
core sample and reaches the downstream side. To validate the proposed
solution, measurements were performed on a core sample of the Cretaceous
Eagle Ford shale, Texas, USA, under different pore and confining
pressures. Helium was used as the test fluid to minimize the
Joule-Thomson effect and adsorption. The experimental results show that
the permeability coefficients obtained from this new solution agree well
with those from the late-time solution, and prove our solution an
accurate and efficient way for permeability evaluation. The present
approach provides a good supplement for the pulse-decay method and
suitable for measurements of ultra-low-permeability rocks.