Numerical Simulation Study On Fracture Propagation Of Hydraulic Fracturing For Hot Dry Rock

Hot dry rock resources are a green,low-carbon,clean geothermal energy source.The development and utilization of hot dry rock are of great significance for adjusting energy structure,saving energy and reducing emissions,and improving the environment.The thermal stress effect of the hot dry rock is obvious in the hydraulic fracturing process,and there is not enough research on the role of thermal stress in crack propagation.In the evaluation of hot dry rock capacity,the single evaluation index and the lack of quantitative description on the evaluation of multiple indicators make it necessary to establish a reliable comprehensive evaluation method.This paper is supported by the National Natural Science Foundation of China and the Applied Basic Research Project for Qingdao.Through the establishment of a mathematical model of thermal fluid-solid coupling damage of hot dry rock,the crack propagation is simulated by COMSOL combined with MATLAB secondary development.Based on the discrete crack model,an integrated evaluation method relied on the analytic hierarchy process and fuzzy comprehensive evaluation method provides an optimization way for the development and optimization of hot dry rock hydraulic fracturing.Firstly,the three-axis meso-damage simulation of hot dry rock is carried out.The simulation result show that the increase of confining pressure leads to the increase of the Young’s modulus and peak strength of the macroscopic properties of the rock,and the rock damage is concentrated near the fracture surface.The increase in heterogeneity leads to the destruction of rock failure and the decrease in Young’s modulus and strength.The effect of the loading rate is mainly reflected in the more serious rock damage and the occurrence of multiple fracture surfaces.Secondly,the paper has carried out the simulation of crack propagation of hot dry rock under temperature.The simulation result show that the larger the temperature difference,thermal expansion coefficient and Young’s modulus,the higher the thermal stress of hot dry rock,which causes the rock to be more easily damaged,the fracture pressure will decrease,and the damage near the wellbore will be more serious.The greater the specific heat capacity and thermal conductivity of the rock,the more moderate the rock temperature will change,which is not conducive to the expansion of the mid-crack,resulting in an increase in the extension pressure.The heterogeneity mainly affects the fracture width,which leads to the crack damage area becoming larger.High injection rate and low rock permeability are more likely to form long fracture.Finally,the paper established the evaluation index system and comprehensive evaluation method for hot dry rock development.The optimization results show that the geothermal energy extraction of horizontal wells is better than that of vertical wells.The geothermal energy production of one injection and two production is better than that of one injection and one production.When vertical wells are used to extract geothermal energy,hydraulic fracturing cracks should be arranged in parallel.When the one injection and two production horizontal well is used to extract hot dry rock,the optimized parameters value are 5 fractures,10 D fracture permeability,400 m well spacing and 20 MPa production pressure difference.