Abstract The structures of methane hydrate obtained from water emulsions in oils of four types, n-heptane and n-decane were studied. Surfactant Span 80 was used to stabilize emulsions of water in n-heptane and n-decane. Hydrate synthesis was carried out by two methods, namely rapid cooling of a water-in-oil emulsion saturated with methane and long-term isothermal holding of this emulsion. It was shown that different methods of hydrate preparation may result in formation of gas hydrates with different structures. Rapid cooling of three of these emulsions (in two oils and n-heptane) saturated with methane to a temperature below −35 °C leads not only to the formation of the expected methane hydrate of cubic structure I (sI) but also to the cubic structure II (sII) hydrate. In case of oils, the formation of the hydrates in the emulsions seemed to occur at a temperature below the pour point of the corresponding oil. Experiments were carried out with the cooling rate about 14 °C/min at initial methane pressures near 12, 10 and 7 MPa. More detailed investigation showed that in two of these emulsions (in one oil and n-heptane) only sI hydrate is formed during long-term synthesis at 1 °C and methane pressure of 12 MPa. The formed sII hydrate must be metastable. In the case of the emulsion in second oil, the formation of sII hydrate can be related either to the kinetic factor (the formation of metastable hydrate) or to the presence of propane and butanes in the corresponding oil in rather high concentrations. The reason of the metastable phase appearance in the systems under consideration is most likely to be that Span 80 and some kinds of crude oil can inhibit nucleation of sI gas hydrate at the oil – water interface. Thus, some emulsions saturated with methane can be overcooled to a temperature at which the nucleation of sII hydrate is preferable. The data obtained are of interest to understand mechanisms of gas hydrate inhibition/promotion and may provide fresh insight into the influence of crude oils and surfactants on gas hydrate nucleation in water – oil – gas systems.

中文翻译：

---

在一些油包水乳液中意外形成 sII 甲烷水合物：同一现象的不同原因

摘要 研究了正庚烷和正癸烷四种油中水乳状液得到的甲烷水合物的结构。表面活性剂 Span 80 用于稳定水在正庚烷和正癸烷中的乳液。水合物合成通过两种方法进行，即用甲烷饱和的油包水乳液的快速冷却和该乳液的长期等温保持。结果表明，不同的水合物制备方法可能导致形成不同结构的天然气水合物。用甲烷饱和的这些乳液中的三种（在两种油和正庚烷中）快速冷却到低于 -35 °C 的温度，不仅会导致形成预期的立方结构 I (sI) 的甲烷水合物，而且还会导致立方结构的甲烷水合物的形成。结构 II (sII) 水合物。如果是油，乳液中水合物的形成似乎发生在低于相应油的倾点的温度下。在接近 12、10 和 7 MPa 的初始甲烷压力下，以约 14 °C/min 的冷却速率进行实验。更详细的研究表明，在这些乳液中的两个（在一种油和正庚烷中）中，在 1°C 和 12 MPa 的甲烷压力下长期合成期间仅形成 sI 水合物。形成的 sII 水合物必须是亚稳态的。在第二种油中的乳液的情况下，sII 水合物的形成可能与动力学因素（亚稳态水合物的形成）或相应油中丙烷和丁烷以相当高的浓度存在有关。所考虑的系统中出现亚稳相的原因很可能是 Span 80 和某些种类的原油可以抑制 sI 气体水合物在油水界面的成核。因此，一些用甲烷饱和的乳液可以过冷至优选水合物成核的温度。获得的数据有助于理解天然气水合物抑制/促进的机制，并可能为原油和表面活性剂对水-油-气系统中天然气水合物成核的影响提供新的见解。