Oil & Gas Geology ›› 2021, Vol. 42 ›› Issue (1): 173-185.doi: 10.11743/ogg20210115

• Petroleum Geology • Previous Articles     Next Articles

Thermodynamics and kinetics of water vapor adsorption onto shale: A case study of the Permian Shanxi Formation, Ordos Basin

Wei Dang1,2,3(), Jinchuan Zhang4, Fengqin Wang1,2, Pei Li4, Chang'an Shan1,2, Ruijing Wang1   

  1. 1. School of Earth Sciences and Engineering, Xi'an Shiyou University, Xi'an, Shaanxi 710065, China
    2. Key Laboratory of Tight Oil and Gas Geology of National Petroleum and Chemical Industry, Xi'an Shiyou University, Xi'an, Shaanxi 710065, China
    3. Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan, Hubei 430074, China
    4. School of Energy Resources, China University of Geosciences(Beijing), Beijing 100083, China
  • Received:2020-06-07 Online:2021-02-28 Published:2021-02-07

Abstract:

Adsorption is one of the key mechanisms for the occurrence of connate water and formation water in shale reservoirs. A characterization of water adsorption onto organic-rich shale is of great theoretical and practical significance to tackling such geological and engineering issues as the micro-distribution of water and gas, the mechanisms of shale gas enrichment and the improvement of shale gas recovery. Shale-water vapor isothermal adsorption experiments were therefore combined with some theoretical adsorption models (7 thermodynamic models and 4 kinetics models) to study the fundamental principles of water vapor adsorption on shale. Results indicate that the adsorption/desorption isotherms are showing the typical TypeⅡcurves with hysteresis loops extending to very low relative pressure region, which may be explained by the fact of reluctant dehydration of clay minerals. Two of the models, GAB and Dent, are proven to be the most fitting to the shale-water vapor isotherm curves and reveal a two-stage water molecule adsorption on shale from forming monolayers to multilayers and capillary condensation. With p/p0 less than 0.1, water molecules are mainly adsorbed as monolayer on one site of shale. With p/p0 between 0.1 and 0.8, the site is gradually saturating and more layers start to build upon the first layer, thus forming a secondary adsorption site. With p/p0 greater than 0.8, the first site is almost fully saturated and the adsorption on the secondary site continues in such a rate that capillary condensation of water occurs. Moreover, the negative values of Gibbs free energy change, enthalpy change and entropy change of the adsorption, indicate a spontaneous, exothermic and entropy-reduction process. Thus the double first-order rate model is the most suitable for describing the adsorption process of water vapor on shale. It reveals that the adsorption process can be divided into a surface adsorption dominated by an earlier external diffusion and a pore adsorption dominated by a later internal diffusion, and that the internal diffusion serves to control the adsorption rate of water vapor onto shale.

Key words: thermodynamics, adsorption kinetics, adsorption mechanism, water vapor adsorption, organic-rich shale, Shanxi Formation, Permian, Ordos Basin

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