Understanding of the absolute adsorption behavior of CH₄ on shale is critically important in estimating shale gas storage in shale gas reservoirs. In this work, two approaches are applied to obtain the absolute adsorption isotherms of CH₄ on shale samples. In the first approach, we first measure the excess adsorption isotherms of CH₄ on two shale samples at the temperature of 298.15 K and pressures up to 12.0 MPa. Then, grand canonical Monte Carlo (GCMC) simulations are used to calculate the adsorption-phase density; such density values are consequently applied to calibrate the measured excess adsorption and obtain the accurate absolute adsorption isotherms. As for the second approach, we apply the low-field nuclear magnetic resonance (NMR) method to describe the absolute adsorption of CH₄ on shale. A NMR-based setup is designed to measure the T₂ spectrum distributions in shale samples by injecting CH₄ into dry shale samples. The injecting pressure is set up to 12.0 MPa, which is similar to the conditions used in the excess adsorption measurements. On the basis of the measured T₂ spectrum and the injected molar amount of CH₄, the adsorbed molar quantity of CH₄ can be assessed on the shale samples under specific conditions. We then compare the absolute adsorption isotherms obtained from both methods and evaluate the capability of the NMR approach in determining the absolute adsorption of CH₄ on shale. With GCMC simulations, we find that the calculated adsorption-phase density strongly correlates with the system pressure and temperature. By taking into consideration the adsorption-phase density, the absolute adsorption isotherm is always higher than the measured excess adsorption curves; that is, the measured excess adsorption underestimates the actual adsorption capacity on shale. On the basis of the comparison results, the adsorption isotherms obtained from the NMR method have a good agreement with the corresponding absolute adsorption isotherms after calibrating with the adsorption-phase density; it indicates that the low-field NMR-based setup is a good tool in obtaining the absolute adsorption isotherms of CH₄ on shale.

中文翻译：

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低场核磁共振法测定页岩上CH ₄的绝对吸附等温线

了解页岩气中CH ₄的绝对吸附行为对于估算页岩气储层中的页岩气储量至关重要。在这项工作中，采用了两种方法来获得页岩样品上CH ₄的绝对吸附等温线。在第一种方法中，我们首先测量CH ₄的过量吸附等温线在两个页岩样品上，温度为298.15 K，压力最高为12.0 MPa。然后，采用经典的蒙特卡罗（GCMC）模拟计算吸附相密度。因此，将这些密度值应用于校准测得的过量吸附并获得准确的绝对吸附等温线。对于第二种方法，我们应用低场核磁共振（NMR）方法来描述CH ₄在页岩上的绝对吸附。设计基于核磁共振的装置，通过注入CH ₄来测量页岩样品中的T ₂光谱分布干燥的页岩样品。进样压力设置为12.0 MPa，与过量吸附测量中使用的条件相似。所测量的基础Ť ₂光谱与所注入的CH的摩尔量₄，CH的吸附摩尔量₄可以在特定条件下的页岩样品进行评估。然后，我们比较了从两种方法获得的绝对吸附等温线，并评估了NMR方法确定CH ₄的绝对吸附的能力。在页岩上。通过GCMC模拟，我们发现计算出的吸附相密度与系统压力和温度密切相关。考虑到吸附相的密度，绝对吸附等温线总是高于测得的过量吸附曲线。也就是说，测得的过量吸附会低估页岩的实际吸附能力。在比较结果的基础上，用NMR法测得的吸附等温线与用吸附相密度进行校正后的相应绝对吸附等温线具有良好的一致性。这表明基于低场NMR的设置是获得CH ₄在页岩上的绝对吸附等温线的良好工具。