Study On The Anisotropic Rock Physical Characteristics In Porous Medium Containing Cracks

Understanding the effect of cracks on the elastic properties,the elastic wave anisotropy and the dispersion is of central importance for the interpretation of 4D seismic data,reservoir monitoring,underground storage monitoring,identify the fluid content and fault mechanics.The main purpose of this paper is to quantitatively characterize the elastic properties,the elastic wave anisotropy and the dispersion in porous medium containing cracks,a series of research work has been carried out in order to investigate the stress-dependent elastic properties and anisotropy,porosity-deformation approach,the elastic anisotropy and the dispersion in porous medium containing cracks.The paper introduces the basic theory of seismic anisotropy in porous medium containing cracks,and it analyzes the effect of stress on the elastic properties of isotropic media,and the stress-induced anisotropy is attributed to the anisotropic closure of cracks under stress.In addition,the analytical model of stress-induced anisotropy is derived,and the experimental data is used to verify the model.In terms of characterization of crack-induced anisotropy,the dimensionless fourth-rank tensor and the new fluid indicator are proposed,and it is found that these two parameters can effectively identify fluid types in cracks.The response of the elastic properties and anisotropy of shale to external stresses depends mainly on the direction of the bedding plane relative to the applied stresses.The porosity-deformation approach explains the stress-dependent elastic properties and anisotropy of rocks well.The experimental data is used to verify this theory,which shows that it is applicable to stress-induced anisotropy.Based on the porosity-deformation approach,the stress-dependent elastic properties and anisotropy of shale are studied,and the damage state and microstructure evolution of shale are evaluated,and the identification of fluid types in shale reservoirs is carried out.Meanwhile,the elastic properties and anisotropy of shale under triaxial loading are studied,which provides a new technical method for the analysis of stress sensitivity components and the prediction of shear wave velocity.Assuming that the elastic anisotropy of rocks is only caused by the existence of cracks,the properties of elasticity and anisotropy of cracked rocks in the state of dry and fluid saturation(high and low frequency limit)are studied based on the effective medium theory and the anisotropic poroelastic theory.The study found that the difference between high and low frequency elastic wave velocities depends only on the crack density tensors,the isotropic solid matrix elastic parameters,fluid bulk modulus,and crack aspect ratio.Then,we analyze the influence of crack orientation distributions on the elastic anisotropy and the dispersion,which lays the foundation for the inversion of crack density and the identification of fluid types in porous medium containing cracks.