Matrix Permeability Measurements of Gas Shales: Gas Slippage and Adsorption as Sources of Systematic Error. Letham, Eric Aiden
At pressures less than 4 MPa, matrix permeability of gas shales show a strong dependence upon the pore pressure at which they are measured. This is the result of gas slippage, a process that causes Darcy’s Law to break down during the low pressure flow of gas through porous media. In gas shales, the range of pore pressures where gas slippage is recognizable is larger than in conventional reservoir rocks. This is attributed to the smaller pore systems found in tighter rocks. The type of gas flowing through porous media also influences the matrix permeability, as the size of molecules is one of the factors that control the mean free path of a gas. For this reason, matrix permeability determined using helium as a probing gas will be different than when methane is used. Comparing this difference to the underestimation of matrix permeability resulting from neglecting to correct for adsorption, the latter is found to be negligible. Therefore, when trying to determine the matrix permeability of a rock to natural gas, no condition was identified where using helium as a probing gas would lead to a more accurate result than using methane.
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