Indoor Simulation Experiment Research On Seepage-dissolution Of Shallow Brine Reservoirs In The Qarhan Salt Lake,Qaidam Basin

Potash resources are important strategic resources,and the Qarhan Salt Lake is the main producing area of potash in our country.In recent years,due to intensive mining,the amount of potash resources in the Qarhan Salt Lake has dropped sharply,and there are considerable reserves of low-grade solid potash resources in the shallow brine reservoir of the Qarhan Salt Lake,it has become an important part of filling the gap of potash resources.However,the mining of this part of solid potash resources is difficult,The current mining method is mainly solution leaching mining,which is restricted by many factors,including ambient temperature,solvent type,solvent injection flow rate,and structural characteristics of the brine reservoir.However,the influence mechanism of the above factors on the physicochemical process of the shallow brine reservoir in the salt lake during the seepage-dissolution process is still unclear,which seriously restricts the conversion efficiency of the solid potassium salt liquefaction mining process in the study area.This paper takes the typical borehole cores in the dissolved mining area of the Qarhan Salt Lake as the research object,and conducts one-dimensional core column seepage-dissolution experiments,including variable flow rate experiments,controlled water head height experiment and different solvent experiments(solvent 1 is 5%saturated NaCl solution of MgCl₂,solvent 2 is a solution containing 0.1%K⁺,2.9%Na⁺,3.77%Mg²⁺,0.05%Ca²⁺,15.72%Cl^-,0.13%SO₄^2-).The variation law of core permeability coefficient and salt mineral dissolution efficiency under the control of flow rate,water head height and solvent type was analyzed and discussed.Saturation index of halite and carnallite minerals in variable flow rate experiment,the control head height experiment and the different solvents were calculated by using the saturation index calculation function of PHREEQC software,and the response mechanism of potassium salt mineral dissolution efficiency to flow rate,water head height and solvent type conditions was analyzed through the change rule of mineral saturation index in the exudate.The main conclusions of this study are as follows:（1）In the variable flow rate experiment,the permeability coefficient of the core and the change of the K⁺concentration of the exudate have a correlation with the flow rate.The greater the flow rate,the faster the increase rate of the core permeability coefficient.Near the time node when the flow rate increases,the core permeability coefficient will increase suddenly.The main reason is the pressure dissolution of salt minerals and the migration and reorganization of mineral particles in the core.The K⁺concentration generally showed a decreasing trend,but the K⁺concentration suddenly increased near the time node of changing the flow rate.The possible reason is that increasing the flow rate leads to an increase in osmotic pressure,which connects the original dead-end pores in the core,breaks the original dominant seepage channel,increases the contact area between the solvent and salt minerals,and increases the dissolved amount of potassium salt minerals.（2）The experimental results of constant head and variable head show that as the height of water head increases,the permeability coefficient of the core will also increase,and the K⁺concentration of the exudate will suddenly increase near the time node when the water head height increases.The possible reason is that as the height of the water head increases,the flow rate of the solvent inside the core increases,and the permeability increases due to the dual action of particle migration and mineral pressure dissolution;at the same time,the increase of the water head height will increase the osmotic pressure of the solvent,which can connect some invalid Pores,intensify the phenomenon of particle migration and reorganization,increase the contact area between solvent and minerals,and finally promote the dissolution of potassium salt minerals.（3）The experimental results of different solvent types under the hydraulic condition of variable flow rate and the hydraulic condition of controlled head height show that both solvents can effectively improve the permeability of the core,but the experimental result of solvent 1 is better than that of solvent 2.The analysis results of the correlation of K⁺,Mg²⁺,Na⁺and Cl^-and the increase of K⁺and Mg²⁺concentration show that both solvent 1 and solvent 2 can effectively promote the dissolution of potassium salt minerals,but the positive correlation index of K⁺and Mg²⁺in solvent 1experiment are higher than those of the solvent 2 experiment（the larger the correlation index,the greater the dissolution rate of the potassium salt minerals per unit time）,and at the same time,the maintenance time of the K⁺increment greater than the Mg²⁺increment during the solvent 1 experiment is longer than that of the solvent 2experiment（this stage indicates that the potassium salt minerals can continue to dissolve）,indicating that solvent 1 has a better potassium-dissolving effect.（4）The simulation calculation results of the saturation index of carnallite and halite minerals show that the saturation index of carnallite in the exudate will increase near the time node of increasing the solvent flow rate and water head height,indicating that increasing flow rate of solvent and water head height can promote the dissolution of potassium salt minerals;the simulation calculation results for solvent 1 and solvent 2experiments show that both solvents can effectively dissolve potassium salt minerals and maintain the relative integrity of the halite skeleton,however,the rate of decrease in the saturation index of carnallite minerals was greater in the solvent 1 experiment and the saturation index of the halite mineral is closer to the saturation point“0”,indicating that the solvent 1 has a higher efficiency of dissolving potassium and has a stronger inhibitory effect on the dissolution of the halite skeleton.The calculation results show that solvent 1 is more suitable as the extraction solvent.The software simulation calculation results well corroborate the conclusions drawn from the indoor simulation experiments.This paper believes that during the seepage-dissolution process,increasing the solvent injection rate,increasing the solvent head height,and using a solution with an ion concentration close to solvent 1 will promote the permeability of the halogen reservoir and the dissolution of potassium salt minerals.