Study On Mechanisms Of Hydrate Formation And Agglomeration In Waxy Water-in-oil Emulsion

With the rapid development of offshore petroleum exploration,subsea multiphase transportation technology has gained extensive attention due to its high economy and transporting efficiency.However,it also leads to a series of novel flow assurance issues and challenges caused by the distinctive transporting environment of deep-water.The shearing action caused by the turbulent flow promotes the formation of water-in-oil emulsions.Since water,gas,and oil in subsea pipelines are transported under low-temperature and high-pressure conditions,wax precipitation/deposition and gas hydrate formation may occur simultaneously.The presence of wax could alter the hydrate formation and agglomeration behaviors by accompanying significant challenges to the flow behavior prediction and flow assurance strategy establishment.Therefore,the current study elucidated the influence of wax on hydrate nucleation,growth,and agglomeration systematically to provide an insight into the flow assurance applications with the presence of both hydrates and waxes.Firstly,a high-pressure autoclave equipped with an online viscometer was employed to explore the influence of wax on hydrate induction time in both model oil system and crude oil system.It was found that the hydrate induction time increased with the increasing wax content.Later,the microscopic observations revealed that the water droplet surface could provide nucleation sites for the supersaturated wax molecules to crystallize and to form the wax shell structure.The wax shell structure was considered to be responsible for the inhibiting effect of wax on hydrate nucleation.Secondly,the high-pressure differential scanning calorimetry(HP-DSC)was employed to investigate the ice crystallization and hydrate formation in waxy water-in-oil emulsion systems.Later,the influence of heterogeneous nucleation effect caused by wax crystals on the hydrate nucleation was studied by combining the analysis of emulsion microstructure and ice crystallization behavior in the waxy emulsion.Also,the effect of water cut and wax content on hydrate conversion was analyzed.Later,the influence of wax on emulsion stability has been studied through comparing hydrate conversion rates in different experimental cycles of one sample.It was found that wax crystals could stabilize the emulsion during hydrate formation and dissociation.In addition,the micromechanical force(MMF)apparatus was used to directly study the influence of wax on hydrate interparticle cohesive force.The wax was found to alter the interfacial properties of hydrate particles and hydrate conversion rates,leading to the hydrate cohesion behavior being changed.The hydrate particle was found to promote the wax crystallization and deposition on the hydrate surface.The presence of the wax deposition layer could significantly reduce the cohesive forces(by 88.6%compared to the system without wax)of hydrate particles.Based on the capillary bridge theory and the liquid film extrusion theory,a cohesive force calculating model was established considering the relationship between the thickness of the hydrate quasi-liquid layer and the capillary force.Finally,the influence of anti-agglomerants on hydrate particle morphology and cohesive force was investigated.Also,the cohesive forces at different contacting periods with the presence of wax or anti-agglomerants were measured.Later,the interconnectedness between the composition of n-alkane and hydrate cohesion behavior was revealed.It was found that the n-alkane with longer chain exhibits a stronger influence on the cohesion behavior of the hydrate particles.Additionally,the hydrate cohesion behavior with the presence of both n-alkanes and anti-agglomerants was investigated,illustrating that the change in the composition of n-alkane could affect the efficacy of anti-agglomerants.