| As a kind of abundant and widely distributed renewable clean energy,geothermal energy plays an important role in achieving the goal of carbon neutrality.The development of deep geothermal energy is mainly achieved by building an enhanced geothermal system(EGS)to exploit hot dry rock(HDR)reservoirs.In the process of hydraulic fracturing and heat recovery of enhanced geothermal system,it is faced with the water-cooling shock caused by the rapid heat exchange between high temperature formation and relatively low temperature fluid.The watercooling shock will degrade the mechanical properties of high temperature granite and affect the construction of fracture network and heat production analysis.However,most of the current hydraulic fracturing fracture propagation models and enhanced geothermal system heat extraction models rarely consider the deterioration of rock mechanical parameters caused by water-cooling shock,which has certain limitations.In this paper,the cracking mechanism and heat recovery process of dry hot rock reservoir under water-cooling shock are studied by means of experiment,theoretical analysis and numerical simulation.The main research contents and results are as follows:(1)The uniaxial compression test and Brazilian splitting test of granite after hightemperature and water cooling were carried out.By normalizing and averaging the test results and previous research results,the deterioration law of tensile strength and elastic modulus of high temperature granite under water-cooling shock was obtained.(2)The influence of crack initiation and morphology in rock hydraulic fracturing.The validity of the coupling model is verified by the results of the previous high temperature granite hydraulic fracturing test.Firstly,the mechanism of water-cooling shock in the process of hydraulic fracturing is studied,and the cracking mechanism of hydraulic fracturing of dry hot rock is clarified.Subsequently,the parameter sensitivity of numerical simulation of hydraulic fracturing of high temperature granite under water-cooling shock was studied,and the influence of key factors on crack initiation and propagation was analyzed.It is found that the deterioration of tensile strength will reduce the initiation pressure and easily form crack branches.Therefore,the numerical simulation of hydraulic fracturing must consider the deterioration of mechanical parameters caused by water-cooling shock.The combined effect of tensile strength degradation caused by water-cooling shock,thermal stress caused by cold shrinkage and pore water pressure is the main reason for crack initiation and propagation in hydraulic fracturing.Water-cooling shock can significantly reduce the crack initiation pressure,induce more cracks,and form more complex fracture networks.The higher the initial temperature of the rock,the greater the contribution of tensile strength degradation to the initiation pressure,indicating that the watercooling shock becomes stronger as the temperature increases.(3)Based on the assumption of elastic thin layer and one-dimensional well,the heat extraction model of enhanced geothermal system is constructed,and the productivity of enhanced geothermal system under different fracture characteristics,reservoir characteristics and artificial control parameters is simulated.It is found that in the case of complex coupling,the assumption of elastic thin layer makes it more convenient to consider the opening and closing of cracks.The mechanical properties of bedrock and fracture are deteriorated by water-cooling shock.The decrease of fracture elastic modulus means that the fracture aperture will increase under the same degree of injection water pressure and confining pressure.Lower bedrock elastic modulus corresponds to higher stress sensitivity and more serious bedrock-crack interaction,which will inhibit crack expansion.Comprehensively considering the impact of water cooling on reservoirs and fractures will more reasonably predict the heat production performance of enhanced geothermal systems. |
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