Study On Mechanism Of Thermal Stress Cracking And Exploitation Pattern For Enhanced Geothermal Systems

Geothermal energy has the advantages of green,stable,abundant and reproducible compared with fossil energy sources such as oil and gas.Although there is abundant geothermal energy in hot dry rock,the feature of large depth,low porosity and low permeability make it difficult to use the high temperature resources.Formation of enhanced geothermal systems by artificial fracturing provides the possibility to efficiently extract geothermal energy from hot dry rock.Due to the high temperature environment of the reservoir,the injection of low temperature fluid(relatively low temperature,the ground temperature is about 20°C)into the high temperature formation(greater than 150°C)during the fracturing process,the thermal stress generated by the temperature gradient and the thermal stress generated by the non-uniform expansion of the particles have an important influence on the fracture initiation and propagation.However,the fracture initiation and propagation mechanism in hot rock fracturing under the influence of thermal stress induced by low-temperature is still unclear,which makes it difficult to effectively guide the scheme design of hydraulic fracturing and accurate prediction of heat mining capacity.This subject is to test the permeability,microscopic structure of pore and rock mechanics parameters of the low-temperature induced specimens under high temperature test conditions,and to determine the influence degree of thermal stress cracking on rock physical and mechanical parameters and its seepage increasing mechanism under different treatment conditions.In order to effectively describe the hydraulic fracturing process of hot rocks under the influence of thermal stress,a thermo-hydro-mechanical-damage(THM-D)coupled model is constructed based on meso-damage mechanics,and the coupled model and solution method are verified by the high-temperature hydraulic fracturing experiments.Using the validated model,firstly the mechanism of thermal stress act in hydraulic fracturing process was studied to determine the interaction between the thermal stress induced by low-temperature and the injected water pressure.Subsequently,the numerical simulation of fracturing under the influence of thermal stress was carried out,and the influence of various factors on fracture initiation and propagation was analyzed.Finally,based on the fracture morphology that may formed after high-temperature rock fracturing,based on the local thermal non-equilibrium theory,the fracture network of the enhanced geothermal system was described by the discrete fracture network,the EGS production capacity under different fracture network morphologies,different well-patterns and exploitation parameters was simulated.The results show that due to the low-temperature induced thermal stress,with increase of the rapid cooling treatment temperature,the rock damage is intensified,the seepage capacity is enhanced,and the mechanical properties are reduced.Taking Sample A as an example,the peak stress is attenuated by 28.2%when the treatment temperature is 400°C.Under the same treatment temperature,the effect of thermal stress cracking induced by low-temperature of A-E rock samples is different.The stronger the heterogeneity of rock mineral composition,the more irregular the geometry of mineral particles,and the larger the Young's modulus of rock will make the more intense thermal stress cracking.In the high-temperature hydraulic fracturing experiment,the failure pressure decreases as the raises of rock temperature.Without loading the axial compression,when the rock temperature is 50°C,the rock failure pressure is 16.2 MPa,while when the temperature raised to 200°C,the failure pressure will reduce to 10.5 MPa.During the high-temperature hydraulic fracturing process,both the increasing injection pressure and the low-temperature induced thermal stress act on the matrix with tensile stress.Under the joint action of the injection pressure and thermal stress,the rock begins to damage.The increase of rock temperature,heat transfer coefficient,Young's modulus,and heterogeneity can lead to more complex fracture networks.For fracturing the high-temperature rock with natural fractures,the secondary fracture formed perpendicular to the natural fractures.The fracture conductivity and closure pressure have an important influence on the formation of secondary cracks.The number of damaged elements generated when K_f=360×10^-14 m² is 67.9%higher than that of K_f=120×10^-14 m².The increase in the density of the fracture network increases the working fluid flow path in the EGS and increases the mass flow rate of the production liquid.The intricate network of fractures increases fluid flow and heat transfer,then slows the heat breakthrough.The best heat mining performance can be achieved when the angle between the fracture and the injection-production well is 45°,and the heating rate is the highest.