摘要
有利油气储集的层状沉积地层中广泛存在裂缝,属于含裂缝VTI介质.综合考虑层状介质和裂缝介质的特点,基于几何相似原理,设计并制作了含定向排列裂缝的VTI介质物理模型,实验研究了测量频率、裂缝特征尺寸、裂缝密度对弹性波透射的影响.结果表明,波垂直裂缝面传播时,纵波首波速度随测量频率的增加而线性增大,峰值速度基本不变,单纯由裂缝导致的衰减以良好的乘幂规律降低;纵波波速随裂缝密度、裂缝特征尺寸的增大而线性增大;衰减随裂缝密度增大而增大,随裂缝特征尺寸的增大而减小.当接收探头波长与特征尺寸比为1~12时,纵波首波速度变化显著,处于射线理论与等效介质理论描述的过渡区间.纵横波速比对裂缝特征尺寸变化敏感,可作为典型的裂缝监测参数,也为基于弹性波相应的预测岩石孔隙结构提供了依据.
Cracks are widespread in layered sedimentary formation,which are important reservoir spaces and seepage channels. In macro perspective this feature can be approximately described as VTI media containing cracks.Carbonate reservoirs often covered with overburden shale,resulting in conventional sonic logging data simultaneously influenced by both the VTI media and cracks which will course agreat deal of uncertainty on reservoir prediction based on the elastic wave propagation method.In this paper,based on geometric similarity principle,combining the layered VTI media and cracked media,the physical models containing aligned cracks in VTI media were designed and produced in laboratory. On the basis, the impacts of measurement frequency,crack characteristic size,crack density on elastic wave propagation were analyzed.The experimental results showed that when the wave propagation was vertical to the cracks surface,with the increase of measurement frequency the first arrival velocity of compressional wave increased linearly,while peak velocity was basically unchanged,and the depletion factor which representing attenuation caused by cracks declined exponentially.Velocity increased linearly with the increase of crack density and crack characteristic size. Attenuation increased linearly as crack density increased while decreases with the increasing of crack characteristic size.The ratio of compressional and shear wave velocity is a sensitive parameter to crack characteristic size,which will be a good indicator for crack monitoring.According to the results of processing the received signals by fast Fourier transform method in time domain,the central frequency were gotten to calculate the ratio of wavelength to crack characteristic size（λ/d）.Because the velocity varies significantly,the transition from ray to effective medium approximation occurred over a narrow range of the ratio of wavelength to crack characteristic size（λ/d）at a value ofλ/d = 1 ~ 12.These works showed that actual crack identification should eliminate the influence caused byλ/dand lay a foundation for crack structure prediction in VTI media from elastic wave responses.
出处
《地球物理学进展》
CSCD
北大核心
2015年第6期2498-2504,共7页
Progress in Geophysics
