Rheology Properties of Cyclopentane Hydrate Slurry in the Presence of Wax Crystals

There are several flow assurance problems for the energy development in deep-water, the coexistence of wax and hydrate resulting to multi-solid phase deposition is among the most difficult to overcome. The results of an experimental study on the effect of wax content, water cut, subcooling and static time on the CP hydrate-forming mixtures are reported. The droplet distribution at different wax contents is determined by using microscopic observation method. Using dynamic stress measurements method, a correlation between rheological properties and water cut is obtained. Hydrate nucleation, growth, dissociation, and slurry rheological properties, including critical time, growth time, final viscosity, and yield properties are determined. Results indicated that wax inhibit hydrate nucleation and growth, but the viscosity of hydrate slurry in waxy system is still higher than wax-free system due to the wax-hydrate aggregates contribute more to the viscosity of the system. The structure of hydrate slurry containing wax system is more stable and more difficult to be damaged by shear. Wax increases the hydrate slurry yield stress, and this effect is more significant at higher water cut and longer static times. The yield stress of the hydrate slurry was linearly related to the water cut. 35% was the critical water cut. Below the critical water cut, the slurry had better flow ability and less yield stress. Above the critical water cut, the yield stress increased significantly with water cut. The yield stress increased with increasing static time and decreases with increasing temperature. Nevertheless, the coupling effect of temperature with water cut and wax content was complex. Below 3 wt% wax content, temperature only has a significant effect on the yield stress of slurries with 40% and 50% water cut. Above 3 wt% wax content, temperature has a significant effect on the yield stress of all slurries with a 20–50% water cut. No ice formation in the CP hydrate-forming mixtures at −4°C, and the dissociation temperature of the hydrate decreases as the concentration of CP in the oil phase decreases.

深水能源开发面临诸多流动保障（flow assurance）难题，其中蜡（wax）与水合物（Hydrate）共存引发多固相沉积的问题，当属最难攻克的难题之一。本研究报道了针对蜡含量、含水率（water cut）、过冷度（subcooling）及静置时间对CP水合物生成体系影响的实验研究结果。采用显微观测法，测定了不同蜡含量下的液滴分布特征；采用动态应力测试法，建立了流变特性（rheological properties）与含水率之间的关联模型。此外，本研究还测定了水合物的成核、生长、解离过程，以及浆体的流变特性，包括临界时间、生长时长、最终黏度与屈服特性。研究结果表明，蜡会抑制水合物的成核与生长，但含蜡体系中的水合物浆体黏度仍高于无蜡体系，这是因为蜡-水合物聚集体对体系黏度的贡献更大。含蜡水合物浆体体系的结构更为稳定，且更难被剪切（shear）力破坏。蜡会提升水合物浆体的屈服应力（yield stress），且在含水率越高、静置时间越长的情况下，该效应越显著。水合物浆体的屈服应力与含水率呈线性相关，临界含水率为35%。当含水率低于临界值时，浆体的流动性能更佳，屈服应力更低；当含水率高于临界值时，屈服应力随含水率升高而显著增大。屈服应力随静置时间延长而升高，随温度升高而降低。然而，温度与含水率、蜡含量之间的耦合效应较为复杂。当蜡含量低于3wt%时，温度仅对含水率为40%和50%的浆体屈服应力具有显著影响；当蜡含量高于3wt%时，温度对含水率介于20%至50%之间的所有浆体的屈服应力均具有显著影响。在-4℃的CP水合物生成体系中未生成冰，且水合物的解离温度随油相CP浓度降低而下降。