Molecular Simulation of Methane Adsorption Behavior in Kerogen Nanopores for Shale Gas Resource Assessment
Molecular Simulation of Methane Adsorption Behavior in Kerogen Nanopores for Shale Gas Resource Assessment
复制标题
DOI：
10.2523/iptc-19216-ms
复制
发表时间：
2019-03
期刊：
影响因子：
--
通讯作者：
Jinrong Cao;Yunfeng Liang;Y. Masuda;Hiroaki Koga;Hiroyuki Tanaka;Kohei Tamura;S. Takagi;T. Matsuoka
中科院分区：
文献类型：
--
作者：
Jinrong Cao;Yunfeng Liang;Y. Masuda;Hiroaki Koga;Hiroyuki Tanaka;Kohei Tamura;S. Takagi;T. Matsuoka
本文提出了一种改进的页岩样品甲烷吸附等温线预测方法，该方法采用分子动力学（MD）模拟和真实干酪根模型。我们将模拟结果与实验结果以及基于简单石墨模型的模拟结果进行了比较，并展示了我们的程序如何在大压力范围内创建更精确的页岩样品吸附等温线。以Marcellus页岩样品为例，演示了如何利用MD模拟计算吸附等温线。干气窗选择ⅱ型干酪根分子模型。所构建的大块干酪根模型内部包含介孔（> ~ 2nm）和微孔（≤2nm）。构建了10种不同介孔尺寸的干酪根纳米孔体系。根据模拟系统中甲烷密度分布的特点，可以清晰地划分出游离气、吸附气和吸收气三个区域。结果表明，随着孔隙尺寸的减小，单位孔隙体积的吸附气量增大。这与以前用石墨模型进行分子模拟类似。为了预测实际页岩样品的总吸附等温线，同时考虑了吸附气和吸附气。对于吸附量，根据Marcellus页岩样品的孔径分布对计算出的吸附等温线进行平均。对于小于5nm的纳米孔，我们使用总有机碳（TOC）数据来衡量干酪根体（即微孔内）的吸收贡献。模拟所得的总吸附等温线很好地再现了实验结果。重要的是，干酪根模型克服了先前研究中使用石墨模型预测的困难（即低估高压条件下的吸附）。此外，我们还预测了较高温度下的吸附等温线。随着温度的升高，吸附量减小。我们改进的方法的新颖之处在于，它能够通过考虑干酪根介孔中的吸附和干酪根体中的吸附来预测页岩样品在大压力范围内的甲烷吸附等温线。它可以很容易地用于任何已知孔隙大小分布、孔隙度和TOC的页岩样品。我们注意到，上述结果和结论是由我们简单的假设得出的。可能需要进一步讨论。
In this paper, we present an improved method to predict the methane adsorption isotherm for a real shale sample using molecular dynamics (MD) simulation with a realistic kerogen model. We compare our simulation results both to the experiment and to the simulation results on the basis of a simple graphite model, and show how our procedure leads to the creation of more accurate adsorption isotherms of a shale sample at a wide range of pressure. A Marcellus shale sample was chosen as an example to demonstrate how to calculate the adsorption isotherms using MD simulations. Type II kerogen molecular model was selected for the dry gas window. The constructed bulk kerogen model contains mesopores (> 2 nm) and micropores (≤ 2 nm) inside. Ten different mesopore sizes of kerogen nanopore systems were constructed. According to the characteristics of methane density distribution in the simulation system, three regions can be clearly distinguished, free gas, adsorbed gas, and absorbed gas. We show that the adsorbed gas per unit pore volume increases with the pore size decreased. This is similar to previous molecular simulations with graphite model. For predicting the total adsorption isotherm of a real shale sample, both adsorbed and absorbed gas were considered. For the adsorption amount, the calculated adsorption isotherms were averaged based on pore size distribution of that Marcellus Shale sample. For nanopores smaller than 5 nm, we used total organic carbon (TOC) data to weight the absorption contribution in the kerogen bulk (i.e. inside the micropores). The total adsorption isotherm thus obtained from our simulations reproduced experiments very well. Importantly, kerogen model has overcome the difficulties of prediction using graphite models (i.e. an underestimation of adsorption under high pressure conditions) as documented in previous studies. Furthermore, we predicted the adsorption isotherms for higher temperatures. With the temperature increased, lower adsorption amount is predicted. The novelty of our improved method is that it is able to predict methane adsorption isotherm at a wide range of pressure for a shale sample by considering both adsorption in kerogen mesopores and absorption in kerogen bulk. It can be readily used for any shale sample, where the pore size distribution, porosity, and TOC are known. We remark that the above results and conclusion resulted from our simple assumption. Further discussion might be necessary.