Study On Fracturing Characteristics And Mechanism Of Hydraulic / Supercritical Carbon Dioxide Fracturing Under High Temperature And High Pressure In Granite

Hot dry rock is a kind of clean energy with great prospect.At present,the heat in hot dry rock is mainly extracted by enhanced geothermal system（EGS）.Existing EGS projects use water as fracturing fluid for reservoir stimulation.Compared with water,supercritical carbon dioxide（Sc CO₂）has unique physical properties.When used in hot dry rock reservoir fracturing,Sc CO₂ has the advantages of reducing pump power and improving thermodynamic efficiency.At present,the research on Sc CO₂ fracturing in dry hot rock is insufficient,and the mechanism of Sc CO₂ fracturing is unclear.Therefore,it is of great significance to study the characteristics and mechanism of hydraulic/Sc CO₂ fracturing.In this paper,the hydraulic/Sc CO₂ fracturing test of granite under different temperature and confining pressure is carried out,the effects of fracturing fluid,temperature and confining pressure on granite fracturing are analyzed,and the effects of stress and natural fractures on fracture propagation are simulated by Particle Flow Code.The main research results are as follows:（1）Hydraulic fracturing has short time and high fracture pressure.The injection pressure curve can be divided into three stages,including filling stage,sharp pressurization stage and depressurization stage;Sc CO₂fracturing takes a long time and the fracture pressure is low.The injection pressure curve can be divided into four stages,including pressurization stage,gentle stage,rapid pressurization stage and depressurization stage.（2）There are many fractures formed by hydraulic fracturing in granite,the fracture morphology is more straight,and the fractures mostly pass through mineral particles when expanding;There are fewer fractures formed by Sc CO₂ fracturing in granite,and the fracture morphology is more tortuous.The fractures mostly extend along the edge of mineral particles,and there are branches in some parts.（3）With the increase of temperature,the fracture pressure of hydraulic fracturing decreases continuously,with a large decrease within 150℃and a small decrease after 150℃;The fracture pressure of Sc CO₂ fracturing also decreases continuously,but the decline remains basically unchanged.With the increase of confining pressure,the fracture pressure of hydraulic/Sc CO₂ fracturing increases approximately linearly.（4）The direction of fracture propagation is parallel to the maximum principal stress direction.Natural cracks affect the morphology of fracture,but do not change the overall direction of fracture propagation.The greater the angle between the natural fracture and the direction of the maximum principal stress,the closer the distance between natural fractures and injection hole,and the more the number of fractures,the greater the impact on the fracture propagation and the more complex the fracture network is formed.