The study of the interaction between hydrate formation and wax precipitation in water-in-oil (W/O) emulsions is of great significance for the security of development in deep-water waxy oil and gas fields. Experiments of natural gas hydrate formation in W/O emulsions containing wax crystals were performed in a high-pressure autoclave. The macro-parametric data, including pressure, temperature, hydrate induction time, hydrate growth amount and rate, were compared and analyzed. Results indicated that the stage behavior of hydrate formation process was not affected by the precipitated wax crystals in W/O emulsions. The mass transfer resistance of hydrate nucleation was enhanced in waxy W/O emulsions. Hence, the hydrate induction time was prolonged and could be estimated by a semiempirical crystallization model developed based on the Freundlich adsorption isotherm theory. Meanwhile, the precipitated wax crystals in W/O emulsions affected the porosity of the hydrate shell, leading to a decrease in the average hydrate growth rate, but the total hydrate growth amount increased compared to the emulsified systems without wax crystals. The effect of hydrate formation and dissociation on the wax precipitation was studied, combined with the data analysis obtained from the polarizing microscopic observation. More wax crystals precipitated in the systems after hydrate dissociation compared to the systems without hydrate formation. The fractal box dimension of the precipitated wax crystals was relatively larger affected by hydrate formation and dissociation, implying that the structure of precipitated wax crystals was more intricate.

Natural gas hydrates (NGH) are prone to causing pipeline blockage in flow assurance, attracting considerable attention in the petroleum industry. This work reviews the significant progress in hydrate flow assurance research in China. Gas hydrate structures are briefly introduced to provide a basic understanding, while the application and development of hydrate management strategies in China are summarized. Subsequently, the development and improvement of hydrate phase equilibrium models are presented, which have been widely applied to the practical challenges of flow assurance. Moreover, kinetics research involving hydrate nucleation, growth, and decomposition are summarized, including nucleation mechanisms, induction time, memory effect, hydrate growth at different interfaces, hydrate growth at a microscopic level, and hydrate decomposition under different systems. The current research status of hydrate slurry flow is also analyzed in detail, covering the viscosity and flow resistance of hydrate slurry and the mechanisms of hydrate particle aggregation, deposition, and blockage. In addition, even though the numerical models of hydrate slurry multiphase flow have been sorted out, the accurate quantitative calculations and risk assessments are still in the initial stage, presenting significant room for improvement. Although substantial research progress has been made in China regarding gas hydrate flow assurance, considerable effort should be devoted to further understanding the intrinsic mechanism work to improve the applicability of various models. This review discusses the current developments, existing problems, and future prospects in the various basic hydrate flow assurance fields in China. It aims to provide readers with an overview of hydrate flow assurance research in China, hoping to provide a reference for developing this field.

With the tendency of the offshore petroleum industry moving to the deep-water fields, there are several challenges for the exploration and development of oil and gas with higher paraffin content in the deep-water severe environment, especially the complex flow assurance issues including the coexistence of wax precipitation and hydrate formation. The effects of wax on hydrate slurry viscosity, hydrate nucleation and growth, and hydrate dissociation were investigated in a rheometer. Results indicated that the stage characteristics of viscosity evolution during hydrate formation in waxy and wax-free emulsions were different, and two stages could be observed during the hydrate growth process in the presence of wax. Hydrate slurry viscosity increased with the wax content. The coupled hydrate–wax aggregates were difficult to be broken by the constant shearing force. The shear-thinning property of the hydrate slurry was not affected by the precipitated wax. Hydrate formation was inhibited due to the wax precipitated in the oil phase. Specifically, cyclopentane hydrate critical time and growth time increased with the wax content. The calculated hydrate volume fraction decreased with the wax content based on the suspension viscosity model. It was difficult for a water bridge to form between two hydrate particles during the hydrate-dissociation process with 3.0 and 5.0 wt % wax contents; therefore, no obvious increase in the slurry viscosity was observed when the slurry viscosity decreased during the dissociation process.

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