Study On CO₂ Fracture Mechanism And Heat Transfer Characteristics Of Dry Hot Rock Thermal Reservoirs

The artificial thermal reservoir is not only a key subsystem of deep geothermal utilization system,but also a fissure network thermal storage reservoir created by fracturing technology.Fracturing technology is also the key to construct thermal reservoir.The main methods are hydraulic fracturing,chemical fracturing and blasting fracturing,but the crack distribution and connectivity formed by these methods are not good enough.With the application of mid-deep geothermal energy in domestic and foreign commercial power generation and heating,The fracturing technology and heat storage performance of hot dry rock thermal reservoirs have also received more and more attention.This paper theoretically analyzes the principle and method of CO2 fracturing and the feasibility of fracturing for hot dry rock reservoirs for a single-fracture heat transfer system in double-well reservoirs,and uses numerical simulation methods to deepen the mechanism of fracture heat transfer in reservoirs Through exploration,design parameters suitable for cracking and heat transfer of hot dry rock reservoirs are obtained.Firstly,the method and principle of CO2 used in coal seam,oil and gas well reservoir fracturing are analyzed.By studying the law of the change of carbon dioxide density,viscosity and thermal conductivity with temperature and pressure,the role of special physical properties of carbon dioxide in the process of fracture extension is analyzed,To study the feasibility of its use in dry hot rock reservoir fracturing,and compare with the effect of hydraulic fracturing.Theoretical analysis results show that it is feasible to use CO2 for the fracturing of dry-hot rock reservoirs,and compared with H2O as a fracturing medium,when CO2 is used as a fracturing medium,many and fine fractures will form inside the reservoir.The crack has better connectivity and a larger heat exchange area.Subsequently,the effects of gap width,working fluid velocity,initial temperature of the rock layer,and working fluid type on the heat transfer effect of the gap-rock mass were studied by numerical simulation.The simulation results show that:the gap width,working fluid velocity,initial temperature of the rock layer,and working fluid type all have a greater influence on the heat transfer effect,and the type of heat exchange working fluid has the most significant effect on the heat transfer effect.The specific performance is:on the basis of other factors unchanged,it increases with the increase of the gap width,increases with the increase of the working fluid velocity,and increases with the increase of the initial temperature of the reservoir;working fluid CO2 phase It has a better heat exchange effect than H2O,which is about 1.34 times that of water.Taking into account the economic,construction and other influencing factors,the above parameters still need to be considered in compromise.Through the above analysis and research,we can obtain:when using CO2 as the working medium,the gap width of 0.014mm,the working fluid velocity of 0.0045m/s,and the initial temperature of the 543K reservoir,the system's net thermal recovery rate is the largest.Under the same working conditions,the heat transfer effect of CO2 fracturing reservoir is about 1.28 times that of hydraulic fracturing,and a thermal breakthrough occurs later to extend the service life of the system.The research results can provide support for the cracking and heat transfer process parameter design of the dry hot rock reservoir.