Research Of High Energy Gas Fractures Initiation And Propagation For Horizontal Well In Shale Reservoir

High energy gas fracturing technology can overcome the shortage of water resources to restrict the development of shale gas in China and increase fractures volume in shale reservoir by creating multi-radial cracks around the wellbore,and is conducive to the effective development of shale gas.Considering characteristics of high brittle mineral content,strong heterogeneity and full of natural fractures in shale reservoir and process features of high energy gas fracturing,Experimental simulation technology and numerical simulation technology was used to explore the fractures initiation and propagation pattern of high energy gas fracturing technology in shale reservoir.During the study,high energy gas fracturing process was divided into two stages,stress wave load stage and high energy gas load stage.According to stress wave loading process,shale gas horizontal well high energy gas fracturing initiation model was established to analysis the impact of loading rate,peak pressure and rock mechanical parameters on fractures initiation and the relationship between loading rate and number of fractures by dynamic finite element method.In view of high energy gas flow feature and mass and momentum conservation equations,gas-solid coupling model of shale gas horizontal well high energy gas fracturing initiation was established to research the relation among decline rate of pressure,shale heterogeneity and natural fractures and fracture geometry based on extended finite element method.The results show that the formation of high Young's modulus is easy to generate narrow and long fractures.And the formation of low Poisson's ratio and high tensile strength is easy to form narrow and short fractures.Low loading rate and high peak pressure is conducive to the growth of fractures.The greater the loading rate,the higher the fractures initiation pressure is,and the more fractures form.Peak pressure has not effect on fractures initiation pressure and number.Decline rate of pressure is a key factor to influence fractures propagation,and decreased decline rate is in favor of fracture propagation.In the process of main-fracture propagation,bedding of diversitylithology has significant differences for stress and strain change.Bedding of high Young's modulus and low Poisson's ratio is severely affected by main-fracture induce stress and partial complex fracture network is easier to form when stress concentration is relatively serious.When high energy gas fractures meet natural fractures in whatever angle it will occur steering with different degree and steering direction is toward to natural fractures.Steering degree is impacted by azimuth of natural fracture and size of approximation angle.The results have important guiding significance for shale gas horizontal well high energy gas fracturing design,optimization and control parameters,and provide important theoretical value.