Study On The Mechanical Mechanism Of Variable Load Hydraulic Fracturing Technology Promoting The Formation Of Fracture Network In Shale Reservoir

The variable load hydraulic fracturing technology has the advantages of low fracture initiation pressure and the ability to form complex fracture network.Therefore,it has great potential in the field of shale oil and gas reservoir fracturing.In order to optimize the effect of variable load hydraulic fracturing on-site construction,it is important to deepen the understanding of the mechanical mechanism of the fracture network formation promoted by variable load hydraulic fracturing.In consequence,critical issues,including the characteristics of fluid pressure propagation in wellbore,the law of perforation friction resistance,the characteristics of pressure distribution in fracture,the mechanical properties of shale and the initiation and propagation of fractures in variable load hydraulic fracturing need to be addressed urgently.In this paper,based on the physical process of variable load hydraulic fracturing,first,the characteristic and mechanism of fluid pressure propagation in wellbore are studied with transient flow theory and wave dynamics theory;then laboratory experiments and local resistance theory of hydrodynamics are utilized to investigate the perforation friction in variable load hydraulic fracturing;the characteristics and mechanism of fluid pressure distribution in fractures are demonstrated with a fluid flow model in fractures and standing wave theory of wave mechanics;based on the theory of linear elastic fracture mechanics and fatigue fracture mechanics,the characteristics of fracture initiation are investigated,the fatigue propagation in variable load hydraulic fracturing are evaluated by using the extended finite element method and direct cycle theory.The key conclusions and understanding are as follows:(1)The characteristics and mechanism of fluid pressure propagation in variable load hydraulic fracturing wellbore are revealed.The bottom hole pressure wave is the superposition of the pressure wave of input frequency and the pressure waves of the fluid column natural frequencies;the frequency of the bottom hole pressure is controlled by the fracturing fluid pumping form and the input frequency,and the bottom hole pressure amplitude is determined by the input frequency,when satisfying the vibration mechanics resonance theory,the bottom hole pressure has the largest amplitudes;pumping fracturing fluid in the form of pulse pressure and pulse flow produces cyclic bottom hole pressure with constant amplitude constant mean pressure and constant amplitude rising mean pressure,respectively.The calculation formula of wellbore liquid column natural frequency can be used to optimize the pumping frequency.(2)The characteristics of perforation friction of variable load hydraulic fracturing are clarified,and the distribution characteristics and mechanism of fluid pressure in fractures are quantified.The perforation friction of variable load hydraulic fracturing decreases with the increase of the number of perforations and the diameter of the perforation,and increases with the increase of flow rate.In variable load hydraulic fracturing,standing waves are formed in the fractures.When the fracturing fluid is pumped in the form of pulse flow,the fluid pressure is distributed evenly;while for pumping in the form of pulse pressure,the fluid pressure unevenly distributed;the frequency of input pressure and fracture properties control the pattern of unevenly distributed pressure;the formation of unevenly distributed fluid pressure in fractures by pumping fracturing fluid in the form of pulse pressure is conducive to the formation of multiple secondary fractures.(3)The damage evolution and fracture mechanical properties of shale in variable load hydraulic fracturing were clarified.The shale damage accumulates rapidly under the action of the constant amplitude rising mean stress cyclic loading;similar to the constant amplitude constant mean stress fatigue characteristics,the constant amplitude rising mean stress fatigue damage variable also develops in three stages,with the difference that the second stage of the constant amplitude rising mean stress damage variable develops in a constant velocity,resulting in a short fatigue life;a nonlinear damage evolution model of shale under the condition of constant amplitude rise mean stress cyclic loading is established.The failure rate of shale samples under cyclic loading increases continuously along the failure path.Cyclic loading causes repeated damage of shale mineral particles,and the failure surface has a large roughness and reduces the fracture toughness of shale.(4)The fracture initiation and propagation characteristics in variable load hydraulic fracturing were evaluated.When the injection of fracturing fluid is controlled in the form of pulse pressure,the stress intensity factor generated by the fluid pressure at the fracture tip is greater than that in the form of pulse flow to control the injection of fracturing fluid.Correspondingly,the fracture initiation pressure is lower than that of conventional hydraulic fracturing,and the fracture initiation pressure increases with the perforation angle,and the fracture initiation angle first increases and then decreases with the increase of the perforation angle.The injection form of fracturing fluid and the horizontal in-situ stress ratio affect the fracture propagation path.The injection of fracturing fluid in the form of pulse pressure induces the development of secondary fractures and the fracture propagation path is easily deflected,which is conducive to the formation of fracture network.The research results of this paper explain the characteristics and mechanical mechanism of fluid pressure in the bottom hole and fractures of variable load hydraulic fracturing.It is shown that variable load hydraulic fracturing technology can cause accelerated fatigue damage of shale,reduce fracture initiation pressure and facilitate the lateral direction of main fractures.The initiation and expansion of branch fractures promote the formation of fracture network,which is expected to provide a theoretical basis for optimizing the effect of variable load hydraulic fracturing.