Experimental Study On Hydrate Slurry Flow And Plugging Characteristics In Waxy System

During the development of deep-water oil and gas fields,the high-pressure and low-temperature transportation environment of gathering pipelines provides suitable conditions for wax deposition and hydrate formation,and will face both hydrate deposition and wax deposition problems,which prone to plugging and threatens the safety of pipeline flow.At this stage,relevant scholars have conducted extensive research on the characteristics of waxy system hydrate slurry flow and plugging,but these studies are mostly based on water-in-oil systems,and studies based on oil-water dispersion systems and oilin-water systems are not sufficient,it is urgent to reveal the influence of wax crystal precipitation on the flow,rheology and plugging characteristics of the hydrate slurry in the oil-water dispersion system and the oil-in-water system.This paper explores the influence of wax crystal precipitation on the flow,rheology and plugging characteristics of hydrate slurry under oil-water system and oil-in-water system.This study is summarized as follows:1.This paper uses a high-pressure visual experimental loop to conduct a waxy oil-water system(no emulsifier added)hydrate slurry flow experiment.The results show that:(1)Compared with the wax-free oil-water system,the waxy oil-water system has a greater downward trend in hydrate flow,a higher pressure drop,and a smaller volume fraction;within the range of 0.5 wt.%～2.5wt.% of wax content,the inhibitory effect of wax crystal precipitation on the formation of hydrates increases with the increase of wax content.(2)Within the range of 0.5 wt.%～1.75 wt.% of wax content,the higher the wax content,the greater the drop in the flow rate of the corresponding system,and the greater the risk of plugging.(3)Synthesizing the plugging morphology image,and aiming at the experimental loop used in this paper,the plugging mechanism of the hydrate slurry under the waxy oil-water system was proposed: after a large amount of hydrate is formed,hydrate particles and wax crystals form aggregates and flow in the form of a suspended phase in the oil-water mixed phase and the water phase with the pipe flow,which causes the fluid viscosity to increase and the system flow rate to decrease,which further reduces the carrying capacity of the pipe flow.The generated hydrate particles and the precipitated wax crystals adhere to each other and large-size aggregates were implanted and deposited,which greatly reduces the flow area of the pipeline,significantly reduces the fluidity of the system,and pipe plugging eventually occurred.2.In this paper,a high-pressure visual experimental loop was used to carry out the flow experiment of hydrate slurry in waxy oil-in-water system.The results show that:(1)Compared with waxy oil-water systems,waxy oil-inwater systems have a smaller flow rate drop,a smaller increase in pressure drop,and a larger volume fraction of hydrate formation.(2)From the plugging diagram,it can be found that there is a hydrate layer on the upper part of the pipeline,and the hydrate particles adhere to each other.A large number of colorless and transparent hydrate layers appeared in the middle and lower parts and gathered at the bottom.Large-sized aggregates appeared and deposited at the bottom of the pipeline,but the size and number of aggregates were different from waxy oil-water system.The reason for the difference is that the aggregation inhibition effect of Tween 80 inhibits the aggregation between particles,resulting in the number and size of aggregates are smaller than the waxy oil-water system.(3)Based on the comparative analysis of the hydrate volume fraction and flow reduction degree of waxy oil-water system and waxy oil-in-water system.Although the foaming property of Tween 80 enhances the capillary action of the hydrate formation process,the final hydrate volume fraction of the waxy oil-in-water system is greater than that of the waxy oilwater system,but the polymerization inhibition effect of Tween 80 leads to a decrease in the flowability of waxy oil-in-water systems less than waxy oilwater systems.3.Based on the pressure drop and flow data obtained from the highpressure loop experimental system,this paper explores the rheological and viscosity characteristics of waxy systems,and draws the following conclusions:(1)The wax-free oil-water system and waxy oil-in-water system hydrate slurry are pseudoplastic fluids.(2)Regarding the precipitated wax crystals and the formed hydrates as the dispersed phase,the effective volume fraction of the total solid phase was obtained,and then the viscosity model of the waxy oil-inwater system hydrate slurry was established.Based on this model,the viscosity of the hydrate slurry of the waxy oil-in-water system was calculated.The model was verified,and it was found that the model can characterize the increasing trend of the viscosity of the system as the volume fraction of solid phase increases,and the error analysis results show that the average relative error between the predicted value of the model and the experimental data is 6.67%,which has good prediction accuracy.(3)Based on the Herschel-Bulkley empirical model,the rheological equations of the wax-free oil-water system,waxy oil-in-water system hydrate slurries were deduced,and found that as the hydrate volume fraction and solid phase effective volume fraction increase,the degree of deviation of the hydrate slurry from the Newtonian fluid is greater.(4)The apparent viscosity of the wax-free/waxy oil-in-water system was studied.It was found that the apparent viscosity of the two systems increased with the increase of the hydrate volume fraction,and the hydrate slurry table of the wax-containing oil-in-water system was analyzed.The reason why the apparent viscosity of waxy oil-in-water system hydrate slurry is always greater than that of wax-free oil-in-water system at each hydrate volume fraction was analyzed.