| Experimental measurements of the seismoelectric effect show that the electromagnetic field induced by seismic waves is closely related to the physical parameters of the reservoir.Since electromagnetic fields are more sensitive to fluids than elastic wave fields,the converted electrical signals in seismoelectric exploration have the ability to indicate fluid interfaces that are difficult to identify in seismic exploration(such as oil-water interfaces),and have the potential to provide important information for conventional seismic exploration.Additional information.Numerical simulation of the wave field in the seismoelectric effect can deepen the understanding of the seismoelectric response characteristics of different types of reservoirs and deepen the explanation of the seismoelectric experiment phenomenon.At present,the commonly used theories to describe the wave field in the seismoelectric effect include the seismoelectric equation of the pore acoustics approximation and the Pride seismoelectric equation.The former has a certain phenomenological form in theory,and only considers the longitudinal wave field and the electric field,and the applicable frequency is around k Hz.However,due to its simple form,it has been applied in the analysis of actual seismoelectric exploration engineering;the latter is based on first principles.The theory is rigorous and complicated.It fully considers the fast and slow longitudinal wave field,transverse wave field and electromagnetic field.The applicable frequency can reach up to MHz.It is usually used as the basis of theoretical explanation for seismic experimental measurement.This article will focus on these two seismoelectric theories and use the finite element method to simulate the wave field of the seismoelectric effect.In order to analyze the coupling characteristics of the longitudinal wave and the electric field in the seismoelectric effect,this paper developed a dual-grid finite element method to solve the two-dimensional pore acoustics approximate seismoelectric equation in the time domain.A concentrated mass matrix is used to avoid the inversion of the coefficient matrix of large-scale linear equations during the recursive calculation of the sound pressure field.In order to take into account the calculation efficiency and accuracy,the quadratic triangular mesh discretization is used in the calculation of the sound pressure field,and the linear triangular mesh division is used for the potential field.The comparison between the numerical simulation results and the analytical solution proves the effectiveness of the algorithm.Then the seismic-electrical response characteristics of sandstone reservoirs with water-heavy oil interface are analyzed.Numerical simulation results show that the seismic-electric interface response is more sensitive to the fluid interface than seismic waves.Finally,the seismic response of the near-surface bedrock uplift and boulder is analyzed.Electrical interface response characteristics.In order to fully understand the wave field characteristics of the seismoelectric effect,this paper developed a global p-type adaptive finite element method to solve the 2-dimensional P-SV-TM mode Pride seismoelectric equation in the frequency domain.When solving the Biot equation,high-order triangular elements are used to ensure the simulation accuracy of the elastic wave field in porous media,and the combined mass matrix is used to further suppress the numerical dispersion.The numerical simulation results are in good agreement with the Green’s function solution,which verifies the correctness of the finite element algorithm.Since the speed of electromagnetic waves is several orders of magnitude larger than elastic waves,the accuracy requirements can be met by using linear triangular elements to calculate the electric field coupled with elastic waves.Finally,the basic characteristics of the P-SV-TM mode seismic wave field are briefly analyzed.The above-mentioned wavefield simulations are all considered point sources,and the emission source transducer used in the seismoelectric experimental measurement is a finite-size line source(in the case of 2D),so the radiated sound field characteristics of the finite-size seismic source need to be considered in order to make the value The simulation result is comparable to the experimental measurement result.This article first discusses the acoustic field radiated by the transducer in the elastic medium in the rock ultrasonic velocity measurement experiment.A set of 2D-3C(two-dimensional,three-component)rock ultrasonic field finite element simulation program was developed.Based on this numerical simulation program,the dynamic model of the longitudinal wave transducer and the transverse wave transducer radiated in the elastic medium was analyzed.Based on the comparison between the numerical simulation and the experimental results of rock ultrasonic velocity measurement,the influence of the acoustic field radiated by the transducer on the shear wave velocity measurement is quantitatively analyzed.It is found that the edge P wave and its converted and reflected waves,couplant,the parasitic longitudinal vibration of the transducer and the longitudinal vibration dipole phenomenon all contribute to the noise before the first arrival of the transverse wave,and when the diameter of the plunger sample gradually decreases,the edge P wave is affected.The converted and reflected waves will gradually increase the interference to the first arrival of the transverse wave.Finally,the radiated sound field of the longitudinal wave transducer in the seismoelectric experiment is considered,and the seismoelectric reflectivity method is used to realize the numerical calculation of the seismoelectric interface response in the seismoelectric tank experiment.The numerical calculation results are consistent with the seismoelectric experiment measurement results.Good,which shows the validity of the numerical forward modeling method of the seismoelectric interface response in this paper,and further demonstrates the correctness of the Pride seismoelectric theory. |
