Experimental Study On Characteristics Of Particle Migration And Deposition Near Wellbore Based On Capillary Model

Groundwater source heat pump system is a type of utilizing geothermal resources.Because of its good stability,high heat exchange efficiency,and low initial investment,it is widely used in building heating and cooling.Normally,the pumping capacity of the system is greater than the recharged water.This will not only reduce the groundwater reserve,but also increase the stress on the solid skeleton of the aquifer,which will result in the narrowing of the aquifer flow channel,causing ground subsidence,and shortening the service life of the system.100%water recharge is particularly important.The problem of aquifer clogging is often encountered in the actual recharge process,which usually includes physical clogging,chemical clogging,and biological clogging.Among them,the physical clogging is mainly the clogging of suspended particles.Therefore,it is necessary to study the characteristics of particle migration and deposition in porous media and effectively solve the problem of physical blockage in the process of recharging.The relevant research content involves the law of fluid movement in porous media and the law of migration and deposition of suspended particles.In this paper,the capillary model simplifies the fluid flow in the porous medium to the pipe flow,and combines the law of particle migration and deposition in the pipe to study the migration and deposition law of suspended particles in the flow channel.Based on the theoretical basis of capillary model,fluid motion equation,solid-liquid pipeline flow,design and build a particle migration and deposition characteristic test bed to simulate the migration and deposition of suspended particles in the flow channel,and carry out pre-test and formal test research.The test plan was designed by orthogonal test method to explore the migration and deposition law of suspended particles in the flow channel under different suspended particle size,pipe wall roughness,and pressure factors.Research shows:（1）Preliminary test found that the general trend of pressure drop of suspension is less than that of pure water.In the formal test,the suspension hydraulic drop with a median particle size of 17.62μm(D₅₀=17.62μm)is less than or equal to the pressure drop of pure water;when the pressure is 1.8 k Pa,the suspension hydraulic drop with D₅₀=25.12μm is less than the pressure drop of pure water.When the pressure is 2.3 k Pa,the suspension hydraulic pressure drop is greater than the pure water pressure drop.Each test factor has different effects on the suspension hydraulic drop.The suspension hydraulic drop increases with the increase of pressure.Under the same conditions,the tube wall roughness and the particle size of suspended particles have different effects on the suspension hydraulic drop.When the pressure is 1.8 k Pa,the increase of pipe wall roughness makes the suspension hydraulic drop of D₅₀=17.62μm increase from0.88 k Pa to 0.9 k Pa;while the suspension hydraulic drop of D₅₀=25.12μm decreases from 0.85 k Pa to 0.79 k Pa.（2）Preliminary tests found that the Reynolds number of the suspension was less than the Reynolds number of pure water.In the formal test,the Reynolds number of the suspension containing D₅₀=17.62μm and the Reynolds number of pure water tend to be the same;when the pressure is 1.8 k Pa and the tube wall roughness K=0.75 mm,the Reynolds number of the suspension with D₅₀=25.12μm is greater than that of pure water.Reynolds number,and vice versa when the tube wall roughness K=1.65 mm.Various test factors have different effects on the Reynolds number of the suspension.The Reynolds number of the suspension increases with the increase of pipe wall roughness and pressure.The size of suspended particles has a different effect on the Reynolds number of the suspension.When the pressure is 2.3 k Pa and the tube wall roughness K=1.65 mm,the particle size increases from 17.62μm to 25.12μm,the corresponding suspension Reynolds number increases from 2461.61 to 2502.49,and the pressure becomes 1.8 k Pa.The diameter increased from 17.62μm to 25.12μm,and the corresponding suspension Reynolds number decreased from 2192.2 to 2053.39.The existence of particles can suppress the disturbance of water flow,and the suppression effect of D₅₀=25.12μm particles is better than that of D₅₀=17.62μm particles.（3）In the process of suspension fluid flow,the suspension fluid displaces pure water,some particles are deposited,and the other part of the particles move along with the water flow.The particle size range of this part of the deposited particles is always maintained at 10-90μm,and the particle distribution is always a single peak.The particle distribution trend is not affected by experimental factors,and the particle deposition position is random.The particle content of this part of the capillary that flows out of the capillary increases with the movement of the water.The particles with a particle size of less than 50μm can penetrate the porous medium and migrate far away,and the particle distribution is always bimodal.When the particle size is smaller than the pore size,it is proved that the particles near the borehole wall will not cause blockage of the aquifer.In summary,pressure,suspended particle size,and tube wall roughness are important factors that affect the migration and deposition of particles in the flow channel.The combination of capillary model and solid-liquid pipeline flow provides new ideas for studying particle migration and deposition in porous media,provides theoretical guidance for solving physical blockage of aquifers caused by particles,and extends the service life of groundwater source heat pump systems.