原位应力下裂缝性致密砂岩各向异性地震波速及其渗透率关联特征

超深层裂缝性非常规天然气藏的开发在能源领域所占比重越来越大，但目前关于该类型储层在原位应力下各向异性地震波速及渗透率的基础研究仍鲜有报道，一定程度上制约了其高效开发策略的制定进程。以塔里木油田克深2号裂缝性致密砂岩天然气储层为工程背景，利用先进的地球物理成像真三轴测试系统，开展了储层衰竭期间原位应力下完整砂岩与不同倾角裂缝岩样的各向异性地震波速以及各向异性渗透率演化规律实验研究，探讨了压缩波、剪切波、纵横波速比、3个主应力方向渗透率各向异性特性与内在机制，研究了地震波速与渗透率的内在关联特征。研究结果表明：首先，裂缝引起的结构非均质性以及三向不等应力作用下岩石各主应力方向微观结构的变形差异导致了地震波速的各向异性；储层衰竭期间地震波速度均随有效应力的增大近似线性增加，压缩波速度明显大于剪切波速度；纵横波速比随有效应力增大而单调减小，表明剪切波对外部荷载敏感性更强；通过压缩波速和纵横波波速比数据对比，发现剪切波S2速度对裂缝倾角更为敏感。其次，原位应力下各主方向渗透率均表现出显著的应力敏感性和结构（裂缝）敏感性，早期应力大小对储层衰竭中后期渗透率变化具有重要影响，渗透率各向异性随裂缝倾角...

Anisotropic seismic wave velocity of fractured tight sandstone underin-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.