Forward Modeling Of Electromagnetic Response Of Hydraulic Fractures Based On Finite Element Method

Hydraulic fracture characterization is the key to fracturing evaluation and strategy optimization.Electromagnetic fracture monitoring can effectively identify the effective support volume of fractures,and has broad application prospects in hydraulic fracture characterization.Based on the theory of low frequency electromagnetic scattering,the formation fracture model is established by finite element method.In order to solve the multi-scale problem between fractures and strata,the transient boundary condition is introduced in this paper,and the hydraulic fractures are approximately equivalent to a plane,so as to solve the problem that the thickness of fractures is difficult to divide,and the fast forward simulation of hydraulic fracture is realized.Considering the stratified characteristics of the formation,the hydraulic fracture model of stratified media is introduced and the forward simulation is carried out.The results show that multi-layer media has a certain influence on fracture response curve.If it is a symmetrical fracture,when the angle between the fracture and the wellbore is in the range of 25°～90°(90°～155°),the smaller(larger)the angle is,the more tortuous the fracture response signal.If it is an asymmetric crack,the larger the widening angle,the more obvious the asymmetry of the crack response signal.The results of the research provide a theoretical basis for horizontal well hydraulic fracturing detection and evaluation.Conventional coil system is inadequate in judging complex fracture morphology and orientation.In this paper,a new method of oblique transmitting coil and vertical receiving coil is proposed to characterize cracks.Through forward modeling,it is found that the response signal of crack is significantly different from the traditional measurement method: for any thin-faced fracture,the inclination angle of the transmitting coil can distinguish the degree of inclination of the fracture;for the rotation angle of the asymmetric crack,the inclination of the transmitting coil can identify the angles of 0° and 180°,and the declination angle can distinguish the angle between 60°and 120°;for the widening angle of the asymmetric crack,when the inclination angles of the two transmitting coils are complementary,it can be The magnitude of the broadening angle is inferred from the difference in the amplitudes of the two response curves.Therefore,the inclined coil structure system has a certain reference value for characterizing the crack position information.