Finite Element Numerical Simulation Of Seismic Wave Attenuation In Porous Media Saturated With Fluid

Porous media saturated with fluid is usually regarded as a complex physical system with very common relaxation and viscoelastic properties.When local seismic waves pass through porous media containing fluid,due to the obvious stiffness difference between the frame of porous media and the pore or fracture part,the fluid and rock frame will be displaced to varying degrees under the compression of seismic waves.This relative displacement will lead to the attenuation of seismic wave energy and velocity dispersion,this phenomenon is often called wave induced fluid flow(WIFF).Due to the limitation of resolution,conventional seismic exploration cannot effectively image the reservoir in mesoscopic scale and pore structure in micro scale,and it is also difficult to extract fluid related information from seismic signals.Therefore,the finite element simulation method is used to simulate the seismic wave attenuation and velocity dispersion caused by wave induced fluid flow,and the influence on the seismic waveform is analyzed.Based on Biot theory and the built-in mathematical equation interface of COMSOL Multiphysics commercial software,we established the finite element calculation model of quasi-static creep test in time domain.The conventional petrophysical model is used to verify its accuracy and effectiveness.In addition,a finite element simulation method of quasi-static creep test based on LNS equation is established.These two methods are used to study the fluid flow patterns in the connected and unconnected fracture models,and two completely different flow patterns are found,as well as the high sensitivity of seismic wave response to fracture connectivity.Then,we analyze the anisotropy of the connected fracture model,and propose a new concept of "fracture effective compression length" to explain the "effective compression" and "ineffective compression" of seismic waves on fractures.In practical application,we extend the numerical simulation program to threedimensional and combine it with the actual low-frequency rock physics experiment to develop a new simulation method called "numerical stress-strain method".The simulation results reproduce the experimental results well and support the conjecture that there are two attenuation mechanisms in fluid containing porous rocks.