WETTABILITY ANALYSIS IN SHALE ORGANIC PORES AT THE NANOSCALE
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摘要:针对页岩有机质分子模型不能真实表征储层孔隙属性, 纳米有机质孔隙中润湿性难以判别等问题, 进行页岩气藏纳米孔隙中基于真实干酪根有机质模型的微观润湿性研究. 利用分子动力学方法, 建立传统有机质模型以及真实干酪根有机质模型, 从模型可视化、空间密度分布特征以及势能机理等方面分析干酪根孔隙中页岩气与水的润湿行为特征, 同时考虑体系温度、孔隙尺寸以及水桥尺寸对润湿状态的影响. 结果表明, 传统有机质模型由于过于理想的假设, 难以对水在有机质中的润湿行为进行精确描述. 干酪根模型以其复杂的分子结构、多样的元素种类, 在有机质的润湿性刻画中有着更好的表现. 纳米有机质孔隙中的水相分为高密度区域与低密度区域, 其中低密度区域中的水分子分布在气液两相的界面处, 所以此部分水分子受到较弱的氢键相互作用, 使其更容易发生逸散并且更易被干酪根基质的相互作用所捕获, 从而在有机质表面进行吸附. 此行为进而表现为一种水分子亲和干酪根基质的润湿假相, 而水相的高密度区域部分却表现为非润湿的状态.Abstract:In view of the problems that wettability is difficult to distinguish in shale nanoscale organic matter pores and the organic matter model cannot truly characterize the pore properties of reservoirs, molecular dynamics research on wettability in shale gas nanoscale pores based on real kerogen organic matter model is proposed. The simulation of smooth and rough graphene ideal models as well as the actual organic matter model of kerogen are built, and wetting behavior characteristics in kerogen pores are analyzed by using the model visualization, the distribution of the density of space, and the analysis of potential energy. There is also an investigation of the effects of temperature, the size of the pores, and the size of the liquid bridge on the wetting condition. Since the traditional organic matter model is based on ideal assumptions, it is difficult to accurately describe the wetting behavior of water in graphene models. As a result of the complex molecular structure and various element types, the kerogen model is more realistic in characterization of wettability of water phase in the organic pore. The water phase in the organic nanopores presents two types of regions: high density region and low density region. Water molecules in the low density region concentrates on the gas-liquid phase interface, where the hydrogen bond interaction is weaker compared with that in the water bulk phase, indicating that this part of molecules are able to diffuse into the gas phase. In addition, the diffused water molecules can also be easily trapped by the kerogen matrix due to its strong attractive interaction. Afterwards, the water molecules will adsorb on the kerogen matrix, presenting a fake wetting condition from the visualization. However, the water phase in the high density region shows the presence of non-wetting condition, which is more realistic for water in the organic pores.
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Key words:
- molecular dynamics/
- shale nanopores/
- wettability/
- shale gas/
- water
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图 1不同时刻下, 水相在光滑石墨烯壁面润湿性的可视化模型
Figure 1.Snapshots of wetting conditions of water phase on smooth graphene walls at various times
图 2不同时刻下, 水相在粗糙石墨烯壁面润湿性的可视化模型
Figure 2.Snapshots of wetting conditions of water phase on rough graphene walls at various times
图 3不同时刻下, 甲烷环境中, 水相粗糙石墨烯壁面润湿性的可视化模型
Figure 3.Snapshots of wetting conditions of water phase and methane on rough graphene walls at various times
图 4甲烷和水相在粗糙石墨烯壁面中的二维密度分布云图
Figure 4.Two-dimensional density contours of water and methane phases in the rough graphene system
图 5干酪根纳米通道中润湿体系的分子模型: 氧原子(红)、碳原子(青)、氢原子(白)、氮原子(蓝)、硫原子(黄)
Figure 5.Molecular models of the kerogen channel (red) for the wetting study: oxygen (red), carbon (cyan), hydrogen (white), nitrogen (blue), and sulfur (yellow)
图 11(a)水和(b)甲烷在不同水桥宽度下的密度分布; (c)水桥宽度2 nm时, 甲烷和水整体的二维密度云图
Figure 11.Mass density profiles of (a) water and (b) methane at various widths of the water bridge; (c) two-dimensional density contour of entire fluid for the 2 nm water bridge
11(a)水和(b)甲烷在不同水桥宽度下的密度分布; (c)水桥宽度2 nm时, 甲烷和水整体的二维密度云图 (续)
11.Mass density profiles of (a) water and (b) methane at various widths of the water bridge; (c) two-dimensional density contour of entire fluid for the 2 nm water bridge (continued)
表 1流体分子相互作用参数
Table 1.Interaction parameters of fluid molecules
Species Charge ε/(kJ·mol−1) σ/nm O (water) −0.82 0.274 0.3608 H (water) 0.41 0.013 1.098 C (CH4) −0.212 0.054 4.01 H (CH4) 0.053 0.02 2.995 
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