Pressure-Dependent Dispersion And Attenuation Of Fluid-Saturated Fractured Porous Media

Fractured reservoirs are becoming increasingly important for oil and gas exploration.Because fractures may control the permeability of a reservoirs,it is important to describe and characterize fractured zones and fractured properties for oil and gas exploration.Natural rocks contain fractures and pores,and the underground environment in which the fractures and pores are located has an impact on their elastic properties.In fluid-saturated rocks,wave propagation might generate local pressure gradients between these compliant pores and adjacent stiff pores,which results in local fluid flow(Wave-induced fluid flow)between pores of different shapes and sizes.This flow,also known as squirt flow,is accompanied by velocity dispersion and attenuation.Based on Collect & Gurevich's equivalent fractured porous model,we calculate the elastic stiffness matrix of the equivalent medium then calculate the frequency-dependent seismic wave velocity and inverse quality factor.By improving the parameters model of fluid-saturated fractured porous media,it is found that the dispersion characteristic frequency changes.In order to further analyze the factors that influence the dispersion and attenuation of seismic wave velocity.By application of compressive stress engenders the pore structure of fractures change and an increase in elastic moduli,which is often attributed to the closure of compliant grain contacts or microcracks.In this way,the fluid in the pores and fractures is squeezed out and the squirt flow influences the dispersion and attenuation of seismic wave velocity weaken.So that,the velocity of seismic wave increases with the increase of pressure.In addition,cracks are closing preferentially in the direction of the applied stress leads to seismic waves propagating faster and a decrease in squirt-flow-induced dispersion and attenuation in this direction,especially at low frequencies.At higher frequencies,the stiffening effect of squirt flow induces a velocity increase in all directions.Cracks are closing preferentially in the direction of the applied stress,which will lead to the change of the fracture rearrangement and the anisotropy of the medium is enhanced under the applied pressure.The physical properties of the fluids in the pores vary with the ambient temperature and pressure conditions.According to the result of pore fluid properties changing with pressure and temperature studied by Michael Batzle et al.,the density and viscosity of fluids will increase with the increase of pressure,which has an impact on the characteristic frequency of seismic wave dispersion.Using Chapman's dynamic equivalent medium theory model calculate the frequency-dependent elastic modulus.Then The frequency-dependent seismic waves velocity and inverse quality factors in different fluids under pressure are calculated.It is found that with different fluids the dispersion and attenuation of seismic wave velocity are different.The characteristic frequency of filling gas in medium is high and the difference between oil and water is not obvious.The viscosity of oil is large and it has a strong effect on the dispersion and attenuation of seismic wave velocity.Furtherly analyze the relationship between relaxation time and fluid viscosity,the relaxation time is proportional to fluid viscosity.Changing the viscosity of the fluid can predict the characteristic frequency of frequency-dependent velocity dispersion under the assumption that the rock skeleton property and the rock permeability are constant.It is found by calculation that the relaxation time of different fluid is also different.When the medium contains gas,the relaxation time is the shortest,which corresponds to the highest characteristic frequency in the case of gas,and the relaxation time is the longest in the case of oil.The relaxation time of different fluids decreases with the increase of pressure.When the pressure increases the cracks closing,the time for fluid to recover to the normal state after being affected by seismic waves will also be shortened.The relaxation time is also different at different scales.At the grain scale,the fluid flow between cracks and pores,the characteristic frequency is higher and the corresponding relaxation time is shorter.At fracture scale,the fluid flow between fracture,the characteristic frequency is lower and the relaxation time is longer.