Zhou Juan, Mao Qian, Luo Kai H
Center for Combustion Energy, Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, People's Republic of China.
Department of Mechanical Engineering, University College London, Torrington Place, London WC1E 7JE, United Kingdom.
Energy Fuels. 2019 Jun 20;33(6):5368-5376. doi: 10.1021/acs.energyfuels.9b00392. Epub 2019 Apr 30.
The adsorption characteristics of methane in shales play a critical role in the assessment of shale gas resources. The microscopic adsorption mechanism of methane considering the effect of moisture and especially salinity remains to be explored. In this work, combined molecular dynamics and grand canonical Monte Carlo simulations are conducted to investigate the adsorption behaviors of methane in the realistic kerogen matrixes containing different moisture contents (0-6 wt %) and various salinities (0-6 mol/L NaCl). Adsorption processes are simulated under realistic reservoir conditions at four temperatures in the range from 298.15 to 358.15 K and pressures up to 40 MPa. Effects of the moisture content on methane adsorption capacities are analyzed in detail. Simulation results show that the methane adsorption capacity declines as the moisture content increases. In comparison to the dry kerogen matrix, the reduction in the maximum CH adsorption capacity is as high as 42.5% in moist kerogen, with a moisture content of 6.0 wt % at 338.15 K. The overlap observed in the density distributions of water molecules and decrease in adsorbed methane indicates that the water molecules occupy the adsorption sites and, thus, lead to the reduction in methane adsorption capacity. Besides, the effects of salinity on CH adsorption isotherms are discussed. The salinity is found to have a negative influence on the methane adsorption capacity. The maximum CH adsorption capacity reduces around 6.0% under the salinity of 6 mol/L at 338.15 K. Adsorption of methane in kerogens of constant salinity but different moisture contents are further discussed. Results from the present study show that the moisture content has a greater impact on the adsorption of methane compared to that of salinity. The findings of this study have important implications for more accurate estimation of shale gas in place.
页岩中甲烷的吸附特性在页岩气资源评估中起着关键作用。考虑水分尤其是盐分影响的甲烷微观吸附机制仍有待探索。在这项工作中，结合分子动力学和巨正则蒙特卡罗模拟，研究了甲烷在含有不同水分含量（0 - 6 wt%）和不同盐分（0 - 6 mol/L NaCl）的实际干酪根基质中的吸附行为。在298.15至358.15 K的四个温度以及高达40 MPa的压力下，在实际储层条件下模拟吸附过程。详细分析了水分含量对甲烷吸附容量的影响。模拟结果表明，甲烷吸附容量随水分含量增加而下降。与干燥的干酪根基质相比，在338.15 K下，水分含量为6.0 wt%的潮湿干酪根中最大CH吸附容量降低高达42.5%。观察到水分子密度分布的重叠以及吸附甲烷的减少表明，水分子占据了吸附位点，从而导致甲烷吸附容量降低。此外，还讨论了盐分对CH吸附等温线的影响。发现盐分对甲烷吸附容量有负面影响。在338.15 K下，盐度为6 mol/L时，最大CH吸附容量降低约6.0%。进一步讨论了在盐分恒定但水分含量不同的干酪根中甲烷的吸附情况。本研究结果表明，与盐分相比，水分含量对甲烷吸附的影响更大。本研究结果对更准确地估算原地页岩气具有重要意义。
