Promotion and inhibition effects of wax on methane hydrate formation and dissociation in water-in-oil emulsions

Abstract

Clarifying the effect of wax on hydrate formation and dissociation are important for the safe and high-efficient development of deep-water oil and gas fields. In this study, a series of experiments on the effect of wax on hydrate formation and dissociation in water-in-oil emulsions were systematically studied based on the variation analysis of temperature, pressure, torque, oil–water interface area morphology and water droplet size in emulsions. Microscopic images reveal that wax crystals form shell barriers on the oil–water interface, which increases transfer resistance, and the particle focused beam reflectance illustrates that the wax crystals decrease the water droplet size, which increases the oil–water interface area in emulsions. With increasing wax concentration, the hydrate induction time and growth rate show a bimodal shape because of the dual regulation of wax in emulsions, involving the inhibition effect of the transfer barrier layer and the promotion effect of the increasing water–oil interface area. The inhibition effect on hydrate formation was dominant when the wax concentration exceeds 6 wt%, resulting in a rapid decrease in both the growth rate and conversion ratio with increasing wax concentration. Compared with the bimodal shape of hydrate formation, the inhibition effect of wax on hydrate dissociation is the dominant regulation factor. The duration of complete hydrate dissociation by depressurization increased linearly with the increasing wax concentration, which increased the operation difficulty of hydrate plug removal. These experimental results provide valuable references for hydrate risk assessment and optimization of the project design of flow assurance for deep-water oil and gas fields.