It is a hot issue to study the transport properties of rocks with low permeability(≤10–16 m2) in geological disposal of radioactive waste,underground gas and oil storage tanks and construction of deep cavern groups. Based on the developed low permeability test instrument,gas transport properties of typical Jin-Ping marbles are studied under steady flow of nitrogen. Also numerical simulation of gas flow through marble is performed. Comparisons are made between pseudo pressure method and an exact method which considers Klinkenberg effect in gas flow equation. Fitting results using the exact method show better agreement with laboratory testing results and the transport parameters gained are more convincible. Numerical study and test results both show that:(1) low permeability test instrument can be used to study transport properties of dense rocks under different temperatures and stress conditions;(2) it is necessary to consider Klinkenberg effect in the study of gas transport properties in dense rocks,and the results from numerical study considering Klinkenberg effects agree very well with laboratory test results.
Permeability test methods for porous media can be divided into two types: steady state method and transient method[1–9]. Stormont J. C. and Daemen J. J. K. studied permeability of salt rock with permeability less than 10–17 m2 using gas transient pulse method and used steady flow method for rock with permeability greater than 10–17 m2[3]. E. Spangenberg studied transport properties of low permeability salt rock using transient method and brine was used as flow media [4]. Zhang [5] derived analytical solutions of different permeability test methods and developed a general test instrument. These analytical solutions are all based on water seepage and can not be extended to tests using gas flow, especially for low permeability sample because of the remarkable Klinkenberg effect. In this study, we derived solutions of gas flow equation in two ways: the pseudo pressure method and an exact method considering Klinkenberg effects in gas flow governing equation. A general procedure has also been developed for accurate evaluation of the analytical solutions. The permeability tests on marbles are Fig1 shows tri-axial cell structure of permeability test machine. Two sets of pistons and plugs can be installed into the tri-axial cell. By controlling the gas pressure at inlet boundary and outlet boundary, different gas pressure gradients in the tested sample can be formed. A flow-meter filled with soap bubbles is connected to the outlet valve and gas flux is recorded in reasonable time interval. The designed maximum gas pressure at inlet boundary is 15 MPa. Permeability of rock samples under a wide range of stress conditions (maximum axial stress: 100Mpa; maximum confining pressure: 30MPa) and temperatures (room temperature ~ 90 °C) can be tested using this instrument.carried out
