Abstract: To test the gas permeability of rock under unsteady flow condition, a new method of calculating the gas permeability was proposed in this paper. Klinkenberg’s equation was firstly imported into the partial differential equation of unsteady flow. A numerical solution of this partial differential equation was then generated by using Picard-Newton’s difference scheme. Based on this numerical solution, a new method of searching the optimal absolute permeability and slip factor of rock was proposed by fitting the variation of gas pressure under unsteady flow condition with least-square method. The numerical calculation in this method was proven to be stable, convergent and insensitive to step size and random errors, and the slight variation of absolute permeability and slip factor can be successfully captured. Finally, some granite samples from the surrounding rock of the Beishan exploration tunnel, which was located in the pre-selected area for China’s high-level radioactive waste disposal, were tested and the absolute gas permeability and slip factor were successfully identified by the proposed method. The input data needed in the proposed method can be easily obtained by simple indoor test and the key algorithm is convenient for numerical realization. Thus it can be applied to the gas permeability test of surrounding rock materials in underground engineering.

Key words: unsteady gas flow, absolute permeability, slip factor, granite