Thermodynamic effect of surfactants on the inhibition of methane hydrate formation in different oil-water system
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摘要:
在天然气水合物开采过程中，水合物分解产生的甲烷以及储层内原位游离气，在合适的低温和高压条件下易重新形成水合物，导致泥水分界线附近高压井筒或管道的堵塞，从而带来安全隐患。为了避免这种情况的发生，使用表面活性剂作为水合物抑制剂，以有效防止水合物的再生成。本文利用高压微量热仪研究了低剂量阳离子表面活性剂十二烷基二甲基苄基氯化铵（DDBAC）对不同油水分离体系中甲烷水合物生成与分解过程的影响。研究选用了四丁基甲基乙醚(TBME)-H2O、甲基环己烷(MCH)-H2O、环戊烷(CP)-H2O体系及对比实验纯水。实验结果表明，低剂量表面活性剂对纯水中甲烷水合物的生成量影响较小，但对水合物结构有一定影响，更倾向于只生成I型甲烷水合物。然而，在TBME-H2O、MCH-H2O、CP-H2O油水分离体系内，低剂量表面活性剂对甲烷水合物及甲烷混合水合物的生成与分解过程具有显著的抑制作用。表面活性剂减小了甲烷在油相中的溶解度，大幅度降低了水合物的生成量，使得TBME、MCH体系易生成纯甲烷水合物，CP体系生成结构更为复杂的混合型水合物。此外，表面活性剂的抗团聚性使得大分子不容易进入笼型，从而不容易形成H型水合物。研究结果对理解低剂量阳离子表面活性剂在油水分离体系中的不同作用机制具有重要意义。
Abstract:In the production of natural gas hydrates, the methane gas released by hydrate decomposition and the in-situ free gas present in sediments are inclined to form or re-form hydrates under a low-temperature high-pressure condition, leading to blockages in high-pressure wellbores or pipelines near the mudline and posing safety hazards. Surfactants can be used as effective hydrate inhibitors to prevent the reformation of hydrates and subsequent blockages. In this paper, we studied the inhibitory effects of low-dose cationic surfactant (dodecyl dimethyl benzyl ammonium chloride) on methane hydrate formation and decomposition processes in different oil-water systems by high pressure microcalorimeter. The systems of TBME (metrabutylmethyl diethyl ether)-H2O, MCH (methylcyclohexane)-H2O, CP (cyclopentane)-H2O , and pure water were selected and tested. Results show that low-dose cationic surfactant has little impact on the amount of methane hydrate formation in pure water. However, it did influence the hydrate structure, favoring the formation of type I methane hydrates. In contrast, low-dose cationic surfactants show significant inhibitory effects on both the formation and decomposition process of methane hydrate and methane mixed hydrate in TBME-H2O, MCH-H2O, and CP-H2O oil-water separation systems. The introduction of surfactants could reduce the solubility of methane in oil phase, and substantially decrease the amount of hydrate formation. Consequently, TBME-H2O and MCH-H2O systems became more likely to form pure methane hydrate, and CP-H2O system is more inclined to form hydrate with more complex structures. Furthermore, the anti-agglomeration of cationic surfactant could prevent large molecules from being captured in the cage structure, thereby inhibiting the formation of H-type hydrates.
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Key words:
- :surfactants /
- oil-water system /
- methane hydrate inhibition /
- thermodynamics study /
- hydrate structure
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表 1 水合物样品反应条件
Table 1. Experimental preparation conditions of hydrate samples
体系 温度/℃ 压力/MPa H2O+DDBAC −10 8 CP5.6mol%- H2O+DDBAC −10 8 MCH5.6mol%- H2O+DDBAC −10 8 TBME5.6mol%- H2O+DDBAC −10 8 
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表 2 不同体系甲烷水合物生成和分解峰的分解温度和分解热
Table 2. The temperature and heat of different methane hydrate formation and decomposition peaks
体系 P/MPa 生成峰 1 生成峰 2 分解峰 1 分解峰 2 T/℃ △H/(J/g) T/℃ △H/(J/g) T/℃ △H/(J/g) T/℃ △H/(J/g) CH4* 8 −7.31 −7.42 10.09 −1.72 10.81 17.20 12.08 2.09 CH4+DDBAC 8 −7.67 −7.46 10.25 −0.65 10.92 18.92 11.18 1.79 CH4-CP* 8 −5.59 −3.98 7.89 −301.56 30.02 312.80 − − CH4-CP+DDBAC 8 −5.16 −113.96 19.43 −0.85 25.04 38.36 29.50 115.30 CH4-MCH* 10 0.54 −21.99 12.32 −276.93 14.57 12.21 17.58 355.56 CH4-MCH+
DDBAC8 −6.46 34.69 10.19 −1.11 10.97 18.86 14.71 49.20 CH4-TBME* 8 −7.54 −16.04 2.27 −184.69 13.41 297.69 − − CH4-TBME+
DDBAC8 3.58 −32.19 9.24 −70.28 10.71 156.18 13.57 8.91 注：*来自文献[13]。
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