| One of the key technologies for exploiting geothermal energy,a kind of renewable green energy with great development value,is to conduct hydraulic fracturing reconstruction on EGS target reservoir.This dissertation carried out an experimental study on the hydraulic fracturing of high temperature rock mass using the“triaxial testing machine of servo control for rock mass at the high temperature of600?and the high pressure of 20MN”independently developed by China University of Mining and Technology.Meanwhile,it established the equation for controlling fracture initiation and propagation process at high temperature,simulated the fracturing process using numerical simulation software,and then analyzed the influence of factors such as the combination of ground stress,temperature and physical and mechanical parameters of rock on the fracturing process.Finally,it drew following conclusions:?1?With large samples of Shandong grey granite being the experimental subject,this dissertation conducted hydraulic fracturing experiments under high temperature and triaxial stress.Through these experiments,it obtained fracture initiation and propagation pressure of the samples at different temperatures and analyzed the reason for the drop of initiation pressure at high temperature.By analyzing fracture propagation patterns,it drew the conclusion that the fracture tends to propagate along radial depth at high temperature.?2?This dissertation studied the mechanism of fracture initiation at high temperature,analyzed the strong transient heat conduction problem by using the non-Fourier equation,and established the thermal shock stress model based on the change of temperature field.It justified the model by using Comsol to solve the equation,analyzing the propagation law of thermal shock stress in rock and making comparison with the experimental results.?3?This dissertation analyzed factors influencing fracture initiation,gave initiation discriminant,analyzed the proportion that various factors accounted for in the initiation pressure based on the experimental data,and obtained the efficiency of changes of thermal stress and tensile strength in the initiation pressure at different temperatures.It is believed that the drop of tensile strength is the main factor controlling the decrease of initiation pressure from room temperature to 200?while thermal stress plays a dominant role after 200?.?4?Though numerical calculation,this dissertation obtained the change law of temperature field and thermal stress field in the rock at different temperatures and analyzed the change of initiation pressure under different geothermal combinations.It is concluded that larger side pressure coefficient is conducive to fracture initiation,and that when the side pressure coefficient gets 1.5 and the temperature reaches 300?,fracture initiation can be achieved just by thermal stress,with no need for the pressurization of fracturing fluid.?5?Through analyzing how the combinations of wellbore azimuth angle with different stresses influence the initiation pressure,this dissertation deemed that the horizontal angle and vertical angle of wellbore have large influence on the initiation pressure,that its sensitivity is related to the side pressure coefficient and that with a unreasonable azimuth angle,higher water pressure is needed for fracture initiation.?6?This dissertation studied the law of fracture propagation of high temperature rock mass under hydraulic pressure effect and then analyzed the change law of the temperature field rock and fluid as well as the effect of thermo-physical property parameter changes on the distribution of water pressure in the fracture by using the governing equation of fluid-solid-heat coupling and the calculation on Comsol software.In addition,after calculating the fracture propagation of rock mass at high temperature by taking the intensity factor of fracture as the discriminant of fracture propagation,this dissertation obtained the change law of propagation pressure at different temperatures and then justified this calculation method by comparing the calculated results with the experimental data.?7?The influence of the dynamic viscosity coefficient of fracturing fluid and the elastic modulus of rock mass on the propagation pressure and the fracture morphology is analyzed in this dissertation.From the analysis,it is believed that the rise of the dynamic viscosity coefficient will increase the propagation pressure of fracture and meanwhile increase the width of fracture.However,the larger the elastic modulus of rock mass is,the smaller the propagation pressure of fracture gets,and meanwhile the narrower the fracture is.?8?In this dissertation,mining simulation was conducted on the geothermal system of a reservoir with double fractures,through which the changes of the temperature field,the fracture deformation and the fracture water pressure in the reservoir were obtained.After analyzing the change of system output with mining time,the dissertation deemed that it conforms to the fitting function y=58.58e-0.08x.After that,it made a recovery calculation on the heat-depleted system.The results show that the recovery ability of the reservoir is much smaller than that in the vicinity of the fracture;besides,the recovery ability of the system is next to nothing in general.Thus,it can be considered that the geothermal system is irrecoverable within a limited time. |
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