An Early-Time Solution of Pulse-Decay Method for Permeability Measurement of Tight Rocks

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 considering the pressure dependence of gas compressibility and Klinkenberg 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 during the early-time stage of the tests, that is, before the upstream pressure pulse penetrates through the core sample and reaches the downstream side. To test 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 testing 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 accurate and efficient for permeability evaluation. The present approach provides a good supplement to the pulse-decay method and is suitable for measurements of low-permeable rocks. Plain Language Summary Unconventional natural gas has become an increasingly important energy source in recent years and has attracted active research and development accordingly. One key problem in unconventional natural gas reservoir exploitation is the determination of the viability of commercial production, where permeability is a critical parameter. The pulse-decay test is the most popular method of permeability measurement for low-permeable rocks. It requires analytical solutions to evaluate the permeability coefficient from pressure records. Most previous methods are based on late-time solutions that interpret the late-time pressure data and omit the information in the early-time stage. The early-time solution developed in this work interprets the early-stage pressure data and considers the variation of gas compressibility and the Klinkenberg slippage effect. The validity and efficiency of the proposed solution are testified by both numerical simulation and experimental measurements.