Study On The Mechanism Of Seepage Force In The Fracture Initiation And Propagation Of Hydraulic Fracturing

Hydraulic fracturing technology is an important technical means to improve oil and gas reservoir seepage and enhance single-well recovery.The progress and development of this technology directly affect the development effect of unconventional oil and gas reservoirs in China.For fracturing technology,the analysis of fracture initiation and propagation is very important for the optimal design of the fracturing operation scheme.A large number of experimental and numerical simulation studies have confirmed that when fluid seepage enters the pores of rock and soil,it will exert a seepage force on the rock and soil skeleton,and the seepage force will break the effective stress balance state of the reservoir rock and have a significant impact on rock and soil damage.However,so far,when the fracturing fluid seeps into the rock pores during hydraulic fracturing,the microcosmic mechanism of the seepage force on the rock unit is still unclear,and the mechanism of the influence of the seepage force on the initiation and propagation of hydraulic fractures is still unclear.Based on this,relying on the National Natural Science Foundation of China,this paper conducts research on the above scientific issues by combining theoretical research and numerical simulation.The purpose of this paper is to reveal the mechanism of seepage force in the initiation and expansion of hydraulic fractures and to provide theoretical guidance for improving the effect of hydraulic fracturing in unconventional reservoirs.The main work and research results of the paper are as follows:(1)By studying the force balance state of the microscopic unit body when the fracturing fluid seeps into the rock pores,the theoretical mechanical model of seepage force was established,and the two action forms of the volume force term and the surface force term when the fluid acts on the reservoir rock alone were analyzed.The influence mechanism of the seepage force term on the effective stress field of the rock skeleton under the control of Biot’s effective stress coefficient was clarified,and the analytical solution of the effective stress field formed by the seepage force around the cylinder under the steady-state seepage condition was deduced.The research results show that when the fracturing fluid seeps into the rock pores,the seepage force cannot be ignored.The greater the Biot effective stress coefficient,the greater the influence of seepage force on the effective stress field of rock skeleton.(2)Using the principle of stress field superposition,the analytical solution of formation breakdown pressure of the open hole and perforated well was deduced considering seepage force,and the mechanism of different factors affecting the formation breakdown pressure of open hole and perforated well under the action of seepage force was studied.Combining the finite difference method and the theoretical mechanical model of seepage force,a numerical simulation model for the dynamic initiation process of hydraulic fractures in the open hole wellbore considering the effect of seepage force was established.The numerical simulation model was used to analyze the influence mechanism of fracturing fluid viscosity and wellbore pressurization rate on the dynamic initiation process and initiation pressure of hydraulic fractures in the open hole wells.The research results show that when the fracturing fluid seeps into the reservoir,the seepage force will significantly reduce the circumferential compressive stress on the well wall and perforation wall,and increase the possibility of tensile fracture in the open hole and perforation holes.The smaller the fracturing fluid viscosity and wellbore pressurization rate,the larger the effective circumferential stress formed by the seepage force,and the smaller the hydraulic fracture initiation pressure.(3)Based on the discrete element particle flow method,a particle-fluid coupling analysis algorithm that can accurately simulate the seepage force was developed.The algorithm is verified in many aspects through the definition of seepage force in soil mechanics,the analytical solution of the induced stress field around the hydraulic fracture and the KGD analytical solution model.Using this algorithm,a numerical model of hydraulic fracturing fracture propagation was established,and the law of artificial fracture propagation in homogeneous sandstone reservoirs and heterogeneous cementation and permeability reservoirs under the action of seepage force was studied.The influence mechanism of fracturing fluid viscosity and displacement on the range of seepage force during hydraulic fracture propagation was clarified.The results show that as the viscosity of the fracturing fluid decreases,the scope of the seepage force exerted by the flowing fluid on the rock particles is significantly enhanced.Under the action of seepage force,the hydraulic fractures will deflect to the side of the weakly cemented high-permeability reservoir and may simultaneously form bifurcated fractures and induced fractures.(4)In this paper,a numerical simulation model for the interactive propagation of hydraulic fractures and natural fractures was established.This model was used to analyze the law of interactive propagation of hydraulic fractures and natural fractures under different natural fracture dip angles,cementation strength ratios,and relative positions of fractures,and the influence mechanism of seepage force on the interactive propagation of hydraulic fractures and natural fractures under different fracturing fluid viscosities.The research results show that the closer the natural fracture is to the main fracture or the weaker the cementation of the natural fracture is,the easier it is to activate and open the natural fracture during the propagation of the artificial fracture.As the viscosity of the fracturing fluid decreases,the seepage force will significantly improve the communication ability between hydraulic fractures and natural fractures.