Study On The Effect Of Fractures On The Heat Storage/release Characteristics Of Buried Pipe Backfill In Deep Mines

With the gradual depletion of shallow mineral resources mining depth in the deepening,the face of the more prominent high well temperature problem,facing the more prominent problem of high well temperature,some domestic and foreign research institutions propose a synergistic mining model of mineral deposits and geothermal,one of the effective ways to achieve geothermal mining is to use functional filling technology to construct buried pipe backfill with the help of existing shaft works in the mine.Under high temperature,high stress and high osmotic pressure environment in deep mines,buried pipe backfill in long-term cyclic heat storage and release operation is bound to fracture damage,causes changes in its heat transfer performance,this in turn has an impact on the efficiency of geothermal extraction,however,there are few relevant studies,the nature of their association is unclear.Therefore,this paper takes the fracture damage buried pipe backfill as the research object,using a combination of experimental tests and numerical simulations,specific research on the effect of fractures on the heat storage and release properties of buried pipe backfill.A multi-field coupled 3D mathematical model of buried pipe-backfill-fracture was established,also designed and built a damage buried pipe backfill heat storage/release test bench,verified the accuracy of the numerical model under no seepage conditions;the accuracy of the fracture seepage model in this paper was verified by using fracture seepage data from the dry heat rock literature.The evaluation index of the performance of buried pipe backfill is given,based on FLUENT simulation software,the effect of a special single fracture on the thermal storage/release performance of an experimentally sized damaged buried pipe backfill under no seepage conditions was investigated,on this basis,the effect of multiple parallel fractures with/without seepage on the temperature field,heat storage/release,buried pipe outlet water temperature and efficiency in the process of heat storage/release in the buried pipe backfill with actual size damage was further analyzed and studied.Firstly,the experimental size damage buried pipe backfill was studied under different boundary conditions,effect of cracking degree,location and width on heat storage/release processes in buried pipe backfill.The results show:no damage to the buried pipe backfill as a benchmark,in case of single-side heating,when the fracture is completely cracked,positioned at X=2cm and width 10mm,minimal heat storage and heat release in buried pipe backfill,by 63.21%and 70.59%,respectively,average heat transfer efficiency reduced by 41.25%;For the same degree of damage to the buried pipe backfill,double-side heating compared to single-side heating,up to 3.22 times more heat storage and 3.67 times more heat release,but compared to non-damaging backfill,the heat storage and heat release decreased by 16.07%and 29.58%,respectively,average heat transfer efficiency reduced by 15.16%.Then the actual size damage buried pipe backfill was investigated with/without seepage flow inside the fracture,effect of the number and spacing of fractures on the heat storage/release performance of buried pipe backfill,When there is no seepage condition,as the number of fractures increases and the fractures spacing decreases,the less heat is stored/released from the damage buried pipe backfill,the damage buried tube backfill with the number of cracks N=7 and the distance between fractures L=10cm stored heat for 8 months and released heat for 4 months,the heat storage and heat release decreased by 11.09%and 10.04%respectively,which is only equivalent to 6.2 months of heat storage and 1.7 months of heat release for the undamaged backfill;The presence of seepage enhances the heat storage/release performance of the buried pipe backfill,the greater the number of fractures and the greater the fractures spacing,the more heat is stored/released from the damage buried pipe backfill,the number of fractures N=7 and the spacing between fractures L=200cm increased the heat storage and heat release by 153.53%and 138.65%respectively,and it only takes 5 days to reach the heat storage of the undamaged backfill for 8 months,while the heat release for 4 months is still higher than the temperature of the undamaged backfill for 8 months.The results of this thesis provide a basis for further work on the effect of irregular fracture network clusters on the heat storage/release properties of damaged backfill at a later stage,to provide a basis for judging the change of heat storage and release performance of buried pipe backfill after long-term operation,a guide to the rational and efficient exploitation of geothermal resources in deep mines.