Study on Solubilization Behavior of Molecular Simulation Cosolvent in Siloxane SC-CO2 Fracturing Fluid
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摘要: 超临界二氧化碳压裂液体系由于黏度低，一般选用加入增稠剂的方法来克服携砂效率低的难题。硅氧烷类增稠剂具有低内聚能和良好的增黏性被广泛地选用，但是使用时需要添加助溶剂提高溶解效果。因此，选用被广泛使用的聚二甲基硅氧烷（PDMS）作为研究对象，利用分子模拟研究了甲醇、甲苯和环己烷等助溶剂的加入对聚二甲基硅氧烷在SC-CO2体系中溶解行为的影响。基于溶剂-溶剂和溶剂-溶质的结合能、径向分布函数和内聚能密度等参数，对比分析了极性助溶剂和非极性助溶剂对聚二甲基硅氧烷在超临界二氧化碳压裂液体系中的助溶效果。分子模拟结果表明，在相同助溶剂含量下，甲醇与溶剂体系溶解度参数差值小于0.5，助溶效果优于甲苯和环己烷。结论分析认为，使用助溶剂提高PDMS在SC-CO2中溶解度的实质是CO2与PDMS聚合物分子间作用力、CO2与助溶剂分子间作用力以及PDMS聚合物与助溶剂分子间作用力的平衡。因此，当硅氧烷类增稠剂本身为非极性材料时，推荐采用甲苯作为助溶剂。若硅氧烷类材料具有一定弱极性时，采用甲醇最为适合。Abstract: Due to the low viscosity of supercritical carbon dioxide fracturing fluid system, the method of adding tackifier is generally used to overcome the problem of low sand carrying efficiency. Siloxane tackifiers are widely used due to their low cohesive energy and good tackifying effect, but it is necessary to add cosolvent to improve the dissolution effect. Therefore, in this paper, the widely used polydimethylsiloxane was selected as the research object, and the effect of the addition of cosolvents such as methanol, toluene and cyclohexane on the dissolution behavior of polydimethylsiloxane in SC-CO2 system was studied by molecular simulation. Based on the binding energy, cohesive energy density and radial distribution function of solvent-solvent and solvent-solute, the solubilization effect of polar cosolvent and non-polar cosolvent on polydimethylsiloxane in supercritical carbon dioxide fracturing fluid system was compared and analyzed. The molecular simulation results show that the solubility parameter difference between methanol and solvent system is less than 0.5 at the same cosolvent content, and the solubilization effect is better than that of toluene and cyclohexane. The conclusion is that the essence of using cosolvent to improve the solubility of PDMS in SC-CO2 is the balance of the intermolecular force between CO2 and PDMS polymer, the intermolecular force between CO2 and cosolvent, and the intermolecular force between PDMS polymer and cosolvent. Therefore, when the siloxane tackifier itself is a non-polar material, toluene is recommended as a cosolvent. When the siloxane material has a certain weak polarity, methanol is the most suitable.
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Key words:
- SC-CO2 fracturing fluid /
- PDMS /
- Molecular simulation /
- Cosolvent /
- Viscosities
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表 1 分子动力学计算系统的构成
体系 组成 PDMS
分子链数助溶剂
分子数CO2
分子数1 CO2 0 0 1000 2 甲苯 0 100 0 3 甲醇 0 100 0 4 环己烷 0 100 0 5 CO2/甲苯 0 100 1000 6 CO2/甲醇 0 100 1000 7 CO2/环己烷 0 100 1000 8 PDMS 2 0 0 9 CO2/PDMS 2 0 1000 10 CO2/甲苯/PDMS 2 100 1000 11 CO2/甲醇/PDMS 2 100 1000 12 CO2/环己烷/PDMS 2 100 1000 
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表 2 318 K下超临界CO2溶解度参数的力场验证
P/MPa P实验值/ ((MPa)1/2) P模拟值/((MPa)1/2) 误差/% 10 7.7 8.736 13.45 14 12.6 12.256 2.73 20 14.3 13.682 4.32 25 15.0 14.416 3.89 30 15.6 15.207 2.52 40 16.4 15.653 4.55
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表 3 CO2与PDMS的内聚能密度与溶解度参数
体系组成 Evan/(J/m3) Eelect/(J/m3) Eother/(J/m3) CED/(J/m3) δ/(J/m3)1/2 Δδ/(J/m3)1/2 CO2 1.88×108 1.75×107 3.66×106 3.66×108 19.13 0 PDMS 2.34×106 6.08×103 3.52×104 2.39×106 1.545 17.59 CO2+PDMS 2.09×108 1.76×108 4.42×106 3.89×108 19.73 0.60
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表 4 CO2与不同助溶剂的结合能
体系组成 Eall/
kJ/molEcos/
kJ/molECO2/
kJ/molEinter/
kJ/molCO2/甲苯 −911.49 738.75 −490.36 −1159.88 CO2/甲醇 −884.74 654.40 −909.02 −1139.36 CO2/环己烷 −1845.68 −510.13 −502.59 −2858.40
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表 5 CO2与不同助溶剂的内聚能密度和溶解度参数
体系组成 Evan/(J/m3) Eelect/(J/m3) Eother/(J/m3) CED/(J/m3) δ/(J/m3)1/2 Δδ/(J/m3)1/2 CO2 1.88×108 1.75×107 3.66×106 3.66×108 19.13 0 甲苯 1.43×107 9.06×105 3.25×105 1.56×107 3.94 15.19 甲醇 0.33×108 2.99×108 4.22×104 3.32×107 5.76 13.37 环己烷 1.21×107 1.35×104 2.53×105 1.24×107 3.52 15.61 CO2+甲苯 2.04×108 1.08×108 4.15×106 3.16×108 17.77 1.36 CO2+甲醇 1.63×108 1.66×108 3.41×106 3.33×108 18.24 0.89 CO2+环己烷 1.73×108 8.25×107 3.53×106 2.59×108 16.09 3.04
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表 6 SC-CO2、PDMS与不同助溶剂混溶体系的结合能
体系组成 Eall/kJ/mol ECO2/kJ/mol EPDMS/kJ/mol Ecos/kJ/mol Einter/kJ/mol CO2+PDMS −4613.80 −1412.85 −3011.84 0 −188.12 CO2+PDMS+甲苯 −3937.61 −317.64 −2973.87 682.27 −1328.37 CO2+PDMS+甲醇 −3995.30 −769.23 −2984.61 584.92 −826.38 CO2+PDMS +环己烷 −4854.87 −416.01 −2956.41 −521.16 −964.29
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表 7 SC-CO2、PDMS与不同助溶剂混溶体系的结合能密度与溶解参数
体系组成 Evan/(J/m3) Eelect/(J/m3) Eother/(J/m3) CED/(J/m3) δ/(J/m3)1/2 Δδ/(J/m3)1/2 CO2+PDMS 2.09×108 1.76×107 4.42×106 3.89×108 19.73 0 CO2+PDMS+甲苯 2.02×108 1.09×108 4.32×106 3.14×108 17.73 2.00 CO2+PDMS+甲醇 1.83×108 1.83×108 3.99×106 3.70×108 19.25 0.48 CO2+PDMS+环己烷 1.79×108 8.50×107 3.79×106 2.68×108 16.37 3.36
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