不同尺度裂缝对弹性波速度和各向异性影响的实验研究

Velocity and anisotropy influenced by different scale fractures:Experiments on synthetic rocks with controlled fractures

裂缝广泛分布于地球介质中并且具有多尺度的特点,裂缝尺度对于油气勘探和开发有着重要的意义.本文制作了一组含不同长度裂缝的人工岩样,其中三块含裂缝岩样中的裂缝直径分别为2 mm、3 mm和4 mm,裂缝的厚度都约为0.06 mm,裂缝密度大致相同(分别为4.8%、4.86%和4.86%).在岩样含水的条件下测试不同方向上的纵横波速度,实验结果表明,虽然三块裂缝岩样中的裂缝密度大致相同,但是含不同直径裂缝岩样的纵横波速度存在差异.在各个方向上,含数量众多的小尺度裂缝的岩样中纵横波速度都明显低于含少量的大尺度裂缝的岩样中纵横波速度.尤其是对纵波速度和SV波速度,在不同尺度裂缝岩样中的差异更明显.在含数量多的小尺度裂缝的岩样中纵波各向异性和横波各向异性最高,而含少量的大尺度的裂缝的岩样中的纵波各向异性和横波各向异性较低.实验测量结果与Hudson理论模型预测结果进行了对比分析,结果发现Hudson理论考虑到了裂缝尺度对纵波速度和纵波各向异性的影响,但是忽略了其对横波速度和横波各向异性的影响.

We investigate seismic velocity and anisotropy in fractured media with different fracture sizes. Using synthetic rocks with controlled fracture geometries, we create a set of rocks with fracture sizes 2 mm, 3 mm and 4 mm, and fracture densities 4. 8%, 4. 86% and 4. 8%, respectively. Fracture thickness is 0.06 mm. P and S wave velocities are measured at 0.5 MHz while the rock samples are saturated by water. The measurements show seismic wave velocity varies in rocks with different sizes of fractures. Velocity is higher in rocks with large-size fractures, while anisotropy is higher in rocks with small-size fractures, even for the same fracture density. The measured data are compared to theoretical predictions generated by the Hudson＇s model. The comparison show the Hudsonrs model considers the effects of fracture size on P wave velocity and anisotropy, but the effects on S wave velocity and anisotropy are neglected. The experimental results in our study demonstrate the important effects of fracture scale, which is neglected by some equivalent medium theories and ignored during seismic wave inversion. We suggest that it is important that these scale effects should be considered when using shear wave splitting to analyse the fracture distribution in earth＇s crust and upper mantle, estimating seismic wave velocity from seismic data or VSP （Vertical Seismic Profile） data, monitoring hydraulic fracturing based on microseismic data, or other fields involving seismic anisotropy caused by fractures. By considering the scale effects of fractures, the accuracy would be improved in these processes.