Predicting Surface Diffusivities of Gas Molecules in Shale,Energy & Fuels

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Predicting Surface Diffusivities of Gas Molecules in Shale
Energy & Fuels ( IF 5.2 ) Pub Date : 2020-09-23 , DOI: 10.1021/acs.energyfuels.0c02441
Dimitris Spanakos ₁ , Sean P. Rigby ₁

Affiliation

Carbon dioxide injection can be utilized as a means of both enhancing gas recovery from shales and sequestering carbon and thereby simultaneously addressing the growing worldwide gas demand, as well as the challenge of greenhouse gas emissions. Greater mobility of CO₂ within the shale improves the displacement efficiency of the originally present CH₄, as well as increasing the CO₂ penetration of the shale formation. Previous investigations have indicated that surface diffusion is much more significant than the bulk gas transport in shale gas reservoirs because of the larger fraction of the adsorbed phase found in the nanopores of shales. The surface diffusivities of CO₂ on different shales, at various temperatures, have been measured. A fractal theory for predicting the Arrhenius parameters of the surface diffusivity of molecules on heterogeneous surfaces has been applied to the surface diffusion of CO₂ in shales. In line with the theory, it was found that both the pre-exponential factor and the activation energy are functions of the surface fractal dimension. Hence, the surface diffusivity, around a monolayer coverage, on shales could be established from an equilibrium gas adsorption isotherm, once the Arrhenius parameters have been calibrated for the specific chemical species. To the best of our knowledge, this study is the first to apply the fractal theory and effectively predict, a priori, surface diffusivity parameters for such structurally and chemically heterogeneous natural samples as shales. This theory now enables the optimization of the designs of CO₂ injection in field applications since surface diffusion is of major importance in the apparent permeability and, thus, in the gas flow mechanisms.

中文翻译：

预测页岩气分子的表面扩散率

注入二氧化碳既可以用作增强页岩气回收和封存碳的手段，又可以同时解决全球日益增长的天然气需求以及温室气体排放的挑战。页岩中CO _2的更大迁移率改善了最初存在的CH ₄的驱替效率，并增加了页岩地层的CO ₂渗透性。先前的研究表明，由于在页岩的纳米孔中发现了较大比例的吸附相，因此表面扩散比页岩气藏中的大量气体传输更为重要。CO ₂的表面扩散率在不同的温度下，在不同的页岩上已进行了测量。分形理论可用于预测页岩中CO ₂的表面扩散，该分形理论可预测异质表面上分子的表面扩散率的阿伦尼乌斯参数。根据该理论，发现预指数因子和活化能都是表面分形维数的函数。因此，一旦已经针对特定化学物种校准了阿累尼乌斯参数，就可以通过平衡气体吸附等温线确定页岩单层覆盖率附近的表面扩散率。据我们所知，这项研究是第一个应用分形理论并有效预测先验结果的研究。，结构和化学非均质天然样品（如页岩）的表面扩散系数。由于表面扩散在表观渗透率和气体流动机理中至关重要，因此该理论现在可以优化现场应用中的CO ₂注入设计。

更新日期：2020-10-16

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