Study On Effect Of Thermo-hydro-mechanical-chemical Coupling In CO₂-EGS Of Hot Dry Rock

The continuous consumption of fossil fuels has caused serious environmental and climatic problems.In response to the energy crisis and global warming,clean renewable hot dry rock?HDR?geothermal energy has gradually attracted widespread attention.Traditional hot dry rock enhanced geothermal system uses water as the working fluid for heat extraction,but it has the disadvantages of huge water resource loss and large energy consumption of circulating pumps.Using CO₂ as an alternative heat extraction fluid in the enhanced geothermal system?CO₂-EGS?has the potential to couple heat extraction and CO₂ geological storage,and the determination of the thermal-hydraulic-mechanical-chemical?THMC?coupling mechanism is the key to improve the heat extraction rate and achieve CO₂ geological storage.In this dissertation,the self-designed“KDQH-60 high temperature and high pressure seepage system”was used to study the seepage characteristics and supercritical CO₂?SCCO₂?heat extraction rate of fractured granite.With the help of a series of observation and analysis methods?including three-dimensional microscope,CT scanning,scanning electron microscope,X-ray diffractometer and ion chromatography?,a comprehensive study of the THMC coupling effect in granite has conducted under high temperature and high pressure from the Meso and micro scales.The main study results are as follows:?1?The uniaxial strength of granite rock samples after none fluid injection,SCCO₂ injection and water-SCCO₂ alternating injection at 150°C-300°C and the uniaxial strength at room temperature were tested.The failure mechanism as well as elastic modulus change with rock sample temperature and fluid injection patterns was analyzed.The results show that after none fluid injection,SCCO₂ injection,and water-SCCO₂ alternating injection,the peak strength and peak strain of the rock sample gradually decrease as the temperature of the rock sample increases.?2?The permeability of rock samples under the influence of pressure and temperature was tested.The results show that the permeability increases with the increase of SCCO₂ injection pressure,but decreases with the increase of confining pressure,rock sample temperature and injection temperature.Based on this,the fitting formulas of permeability changes with confining pressure,rock sample temperature,SCCO₂ injection pressure and injection temperature are given.?3?Considering the influence of temperature on the thermal conductivity of rock mass,the calculation relationship of the local average heat transfer coefficient?LAHTC?of fractured rock samples was given.The heat extraction rate of CO₂ under high temperature and high pressure was tested,and the effects of confining pressure,rock sample temperature,SCCO₂ injection pressure and injection temperature on the heat extraction rate and LAHTC were analyzed.The results show that after increasing the confining pressure,rock sample temperature and injection temperature,the LAHTC of the fracture and the net heat extraction rate are reduced,while increasing the injection pressure is opposite.?4?The permeabilities of rock samples with and without proppant are compared and analyzed during the alternating injection cycles.The results show that in both cases the permeability decreases with the increase of injection cycle,but there is no order of magnitude change in permeability after proppant is added and the reduction is much smaller than without proppant.When no proppant is added,the initial decrease speed of permeability is large,and the curve shows a periodical violent-slow-fast-slow trend.After adding proppant,the contact form of the fracture surface is changed,and the permeability decreased slowly during initial period.When the mineral particles in contact with the proppant on the fracture surface are dissolved by pressure,the permeability is slightly reduced.?5?The mesoscopic and microscopic morphology of the fracture surface after water-SCCO₂ alternating injection were observed.The mineral ion content in the fractured water sample and the mineral composition changes on the fracture surface were analyzed.The forms of mineral dissolution and precipitation on the fracture surface under the coupling of THMC were revealed.The results show that after dissolution of feldspar on the fracture surface,carbon storage minerals such as cluster granular and rod-shaped calcite,massive dolomite,and other minerals such as needle cluster calcium zeolite,flaky illite,and scaly montmorillonite are generated.Through the principles of conservation of mass and solution charge balance,the CO₂ storage rate in the heat extraction process was calculated to be 0.0554%.?6?According to the theory of thermal porous elasticity,the stress field model under thermal expansion?cold shrinkage?,the model of reservoir mineral dissolution and precipitation reaction,and the permeability evolution model were established,and the THMC coupling model of CO₂-H₂O two-phase flow was established based on above.The model was used to analyze the influence of SCCO₂ injection pressure and injection temperature on heat extraction and CO₂ storage as well as the variation of reservoir geological liquid ion concentration.The results show that increasing the injection pressure is beneficial to heat extraction,but shortens the production life of the reservoir.Lower injection pressure will produce a higher CO₂ storage rate.Increasing the injection temperature is beneficial to prolong the production life of the reservoir,but is not conducive to heat extraction and will reduce the CO₂ storage rate.There are 125 figures,28 tables and 168 references in this dissertation.