Experimental Study On The Characteristics Of Exogenous Particles Migration Near The Well Wall Induced By Recharging

The groundwater heat pump system is a green air-conditioning system that uses a confined aquife as a cold or heat source to provide heating and cooling to buildings.Its contribution to energy conservation and pollution reduction is outstanding and undubtful,but some negactive influences are compained,among which the difficulty of recharging to medium-fine sand confined aquifer is the key to restricting its sustainable development.The confined aquifer dominated by loose sediments and no-assicoiated particles has poor permeability,which will slow down the injection water.The exogenous particles carried by the injection water will clogging the seepage channel and reduce the permeability.The clogging is common divided into physical,chemical,and biological clogging,and the particle clogging who occurs faster and more serverely accounts for 50%of the total clogging.Therefore,it is particularly important to further study the migration characteristics of exogenous particles in the congfined aquifers near the well wall during recharging.A sandbox bench for suspension injection is established to simulate the injection procedure of the groundwater heat source pump.Four test conditions are designed,two initial injection pressures(17.058 k Pa,14 k Pa)and three matrix particle grading(D_p=1.026 mm,D_p=1.343 mm,and D_p=1.721 mm)were carried out to study migration-sedimentation characteristics of exogenous particles near the well wall induced by recharging,and the selection of filter membrane grade.Research contexts have shown that:(1)The migration of suspended particles in the aquifer near the borewell is affected by mechanical dispersion.The dispersion effect is more significant when the initial injection pressure is higher and the matrix particles are coraser.The specific performance is the penetration time of larger suspended particles at this condiction is relatively delay.(2)Energy loss is existed when the injected suspended particles flow in the well,particles is esay to be filtered by the sieve screen and deposited by gravity in the inlet chamber and the aquifer near the well wall.The suspended particles are easier deposition when the matrix particles is finer.(3)The overall distribution of the injection pressure tends to beconsistant in each working conditions.After the maximum value appears,the injection pressure gradually decreases and finally maintains a certain pressure level.Due to the limited operation,periodic variation of injection pressure is no obvious.In the actual projects,the particle clogging may be periodcally repeated,and the injection pressure should be coordinated with the development of the particles clogging.(4)The particle distribution interval in the outflow suspension is mainly concentrated in the range of 1-30?m.The particle size level is always greater than the current level of groundwater recharge filter membranes of 3?m.It can be considered that it is not necessary to filter the 3-30?m particles carried in the injection water,and the grade of filter membranes can be selectively reduced.Under the premise of ensuring sand control,the design of the hole diameter of the borewell screen should be as large as to allow the abstracted water carrying the sand to pass.(5)The variatioin of the permeability affected by both injection pressure and matrix particle grading..When the initial injection pressure is lower and the matrix particles are finer,the range of permeability affected is larger.The influence of suspension on the permeability is less obvious while the matrix particles become coarser.The migration of the suspended particles in the porous medium is affected by the boundary,which is reflected in the permeability on the left and right sides of the sand box changes in similar trends,and the value is smaller than those at the center of the seepage;the impact of the upper and lower boundaries is not obvious.In summary,the injection pressure and the particles grading of matrix are important factors affecting the migration of exogenous particles near the well.In actual injection projects of confined aquifer with medium-fine sand,a higher injection pressure can be designed to reduce the sedimentation risk of exogenous particles near the well,where is the critical part for rehabilitating the clogging;a reasonable filtration membrane grade can also be selected according to local aquifer conditions.