Anisotropic seismic wave velocity of fractured tight sandstone under in situ stress conditions and its correlation to permeability

The development of ultra deep fractured unconventional gas reservoirs accounts for an increasing proportion of the energy sector. However，few reports on the basic research of anisotropic seismic velocity and permeability of this type of reservoir under in situ stress have been attempted，which limits the process of drafting an effective development strategy. Taking the Keshen 2 fractured tight sandstone gas reservoir in Tarim Oilfield as the engineering background，the advanced geophysical imaging true triaxial testing system is utilized to study the evolution law of anisotropic seismic velocity and permeability of intact sandstone and fractured samples with different dip angles under in situ stress. The anisotropy characteristics and internal mechanism of compression/shear wave，velocity ratio and directional permeability are discussed. Besides，the intrinsic relationship between seismic wave velocity and permeability is also clarified. Experimental results show that，firstly，the initial structural heterogeneity induced by the fracture，together with the deformation difference of the microstructure in each principal stress direction caused by the unequal stresses，leads to the seismic velocity anisotropy. The seismic wave velocity increases approximately linearly with the increase of the effective stress during the reservoir depletion，and the compression wave velocity is significantly greater than that of the shear wave. The compressional to shear velocity ratios decrease with the increase of effective stresses，indicating that the shear wave is more sensitive to external loads. From the comparison of compressional to shear wave velocity ratios and P-wave velocity data，it is found that the velocity of shear wave S2 is more notably influenced by the fracture inclination. Secondly，the directional permeability in each principal direction under in situ stress exhibits significant stress sensitivity and structural (fracture) sensitivity，and the magnitude of the initial stress has a decisive effect on the subsequent evolution of the permeability. The permeability anisotropy becomes more remarkable with the increase of the fracture dip angle. The permeability in the direction perpendicular or oblique to the fracture plane is relatively smooth，whereas the permeability along the fracture plane shows certain fluctuation characteristics，which can be mainly attributed to the fact that due to the dual effect of pressure fluid erosion and the deformation of the rock skeleton under further compression，the newly broken microparticles may continue to migrate within the pore/fracture structures，thereby causing periodic blockage and dredging of the pores/fractures in the rock. Furthermore，under the experimental stress condition (strikeslip stress regime)，the smaller the fracture dip angle，the less obvious the seismic wave velocity permeability correlation in the horizontal principal stress direction. The relationships between the seismic velocity and directional permeability along each horizontal principal stress direction should be treated differently，and the correlation between the velocity and the permeability along the maximum principal stress direction is more significant than that along the minimum principal stress direction.