Natural Gas Industry B (Jun 2020)
Thermodynamic characteristics of CH4 adsorption by continental shale: A case study of the Upper Triassic Yanchang shale in the Yanchang Gasfield, Ordos Basin
Abstract
In order to improve the calculation method of the isosteric adsorption heat, clarify thermodynamic characteristics of CH4 adsorption by continental shale and reveal the adsorption mechanism, this paper selected shale samples from the seventh Member of Yanchang Formation of Upper Triassic in the Yanchang Gasfield of the Ordos Basin as the research object. The isothermal adsorption experiment was carried out on the CH4 adsorption by continental shale and the excess adsorption curves were plotted. Then, the characteristics of the isosteric adsorption heat of the shale with different types of adsorption capacity were illustrated by analyzing and comparing the difference between excess and absolute adsorption capacity. And the following research results were obtained. First, under the same temperature and pressure, absolute adsorption capacity is higher than the excess values. The difference between them is higher under low temperature and high pressure and it is in the relationship of exponential function with the equilibrium pressure. If excess adsorption capacity is used to evaluate the adsorptive property of shale reservoirs, the evaluation result will be underestimated. Second, for CH4 adsorption by Yanchang Formation shale, the absolute and excess isosteric adsorption heat values have a linear positive correlation with absolute and excess adsorption capacity respectively, and the intermolecular force of adsorbate has a dominant effect on isosteric adsorption heat values. Third, absolute isosteric adsorption heat value is less than the excess heat. The relative error is in the range of 18.18–49.79%, and it is higher in the stage with low adsorption capacity. If excess adsorption capacity is taken as the basic data to calculate initial isosteric adsorption heat values, the calculation result will be overvalued, and consequently, the evaluation of the intermolecular force of adsorbent and adsorbate is overestimated.
