Research On Fracture Creation Characteristics And Fracture Propagation Based On Phase Field Model

Tight reservoirs have the characteristics of low porosity,low permeability,and a large number of natural fractures,and hydraulic fracturing technology is generally used for fracturing.However,in the actual fracturing process,because the natural fractures in the reservoir have various development directions,under the influence of natural fractures,sometimes the hydraulic fractures will turn to the direction unfavorable for development,which will reduce hydraulic fractures and natural fractures.Therefore,it is necessary to combine natural fractures to study the characteristics of fracturing and the propagation and extension of hydraulic fractures in the process of hydraulic fracturing.This will improve the hydraulic fracturing effect of tight reservoirs and facilitate effective development.However,the currently used numerical simulation methods such as displacement discontinuity method(DDM),discrete element method and XFEM are difficult to deal with the simulation problems such as crack propagation and intersection.In order to solve the above problems,based on the basic principle of the phase field method,this paper establishes a phase field model of fracture propagation in reservoir fracturing,and uses two methods of numerical simulation and hydraulic fracturing physical model experiment to study the natural fractures and reservoir formation in the process of hydraulic fracturing.Under the influence of stress difference,the characteristics of hydraulic fracture formation and the law of expansion and extension are verified with each other.It is pointed out that during the fracturing process,the direction of perforation can be controlled to control the angle between hydraulic fractures and natural fractures,and the effective communication between hydraulic fractures and natural fractures can be enhanced.Provide scientific guidance for fracturing and stimulation of tight reservoirs.The main contents and achievements of this paper are as follows:(1)Based on the phase field method,based on the variational principle and the principle of energy conservation,a phase field model of hydraulic fracturing fracture propagation in reservoirs is established,and a fracture propagation model of hydraulic fracturing containing natural fractures is established.The influence of fracture and maximum principal stress angle and horizontal in-situ stress difference on the propagation law of hydraulic fractures.The simulation results show that for a tight reservoir with a horizontal in-situ stress difference of8 MPa and an angle of 75° between natural fractures and the maximum principal stress,hydraulic fractures and natural fractures can be effectively communicated,and the hydraulic fracturing effect is the best.(2)Combined with the analysis of the mechanical characteristics of tight reservoirs,the characteristics of fracturing and hydraulic fractures under the condition of different levels of in-situ stress difference and the angular relationship between natural fractures and the direction of the maximum principal stress are studied through the true triaxial hydraulic fracturing model experiment.The extension law is extended to verify the correctness of the numerical simulation results.The experimental results show that: 1、When the horizontal principal stress difference is the same and the angle between the natural fracture and the direction of the maximum principal stress increases,the hydraulic fracture changes from the propagation mode along the natural fracture to the propagation along the natural fracture and through the natural fracture.The expansion mechanism forms a stress+fracture balanced composite fracture expansion mode,which is conducive to the formation of a volumetric fracture network with balanced expansion;2、When the natural fracture is at an angle with the direction of the maximum principal stress,and the horizontal in-situ stress difference increases,the hydraulic fracture in the natural The fractures propagated along the direction of the maximum principal stress,forming a stressdominant composite fracture propagation mode,indicating that the hydraulic fractures were significantly affected by the in-situ stress difference and formed a penetration fracture propagation mode.The research results of this paper reveal the propagation law of hydraulic fractures in hydraulic fracturing of tight reservoirs,and provide a scientific basis for fracturing stimulation.