A method to determine the permeability of shales by using the dynamic process data of methane adsorption

Abstract

Permeability is a critical parameter to evaluate the gas migration and development of shales, however, very difficult to get at the laboratory. Methane adsorption in shales is also an important factor of shales in natural state. A method of permeability that considers methane adsorption is described in this study. Permeability is derived from the dynamic data of the methane adsorption equilibrium process. The carboniferous shale samples, C01, C02, C03 and C04, were extracted from the eastern Qaidam Basin in China. The properties of the shales including organic/inorganic matter contents and pore size distribution were obtained by several techniques. Methane adsorption experiments were conducted on crushed shale samples over a pressure range up to 10 MPa and at a temperature of 313.15 K. Additionally, the permeability was calculated using the dynamic data at the adsorption equilibrium. Correlations between the permeability and methane adsorption and sample properties were discussed. The parameters of the calculation process were also analyzed.A parameter called the methane adsorption rate is defined as θ, which represents the variation in methane adsorption amount with pressure. The permeability is found to have the same variation trend with θ at experimental pressures before the inflection point. The pressure relative to the inflection point decreases with the micropore volume percentage, and the permeability increases with the methane adsorption affinity for the sample, nmax. We found that the permeability before the inflection point is considered to represent the micropores permeation property. The methane adsorption rate decreases to an infinitesimal value until the adsorption reaches a saturated state. The permeability values are assumed will stop fluctuating and become a fixed value, which we call kf. The parameter kf is regarded as the true permeability of the sample. The assumption gives a novel idea of permeability calculation. In addition, the permeability of the samples increases with the macropore volume, and the methane adsorption capacity nmax also increases with the total porosity.