裂缝诱导TTI双孔隙介质波场传播特征

Wavefield propagation characteristics in fracture-induced TTI double-porosity medium

本文在考虑油藏内流体影响的基础上,进一步讨论了裂缝诱导各向异性的极化角和方位角的影响,对裂缝诱导TTI(tilted transverse isotropy)双孔隙介质模型进行了研究.在裂缝诱导HTI(horizontal transverse isotropy)双孔隙介质理论的基础上,用Bond变换推导了裂缝诱导TTI双孔隙介质的柔度系数矩阵和耗散系数矩阵,从而建立了介质的一阶速度应力方程.采用交错网格高阶有限差分法及PML边界条件,对xoz平面内的2.5维矢量波动方程进行了数值模拟.结果表明,裂缝的极化角和方位角的存在都会导致横波分裂,而在双层裂缝诱导TTI双孔隙介质模型的分界面上,又会产生转换波的分裂和横波的再分裂现象,这就增加了波场的复杂性,从而为进一步研究实际地球介质的地震波场传播特征奠定了基础.

The direction of symmetry axis of parallel fracture set in fractured hydrocarbon reservoir affects the transmission of seismic waves markedly, so a medium named fracture-induced TTI （tilted transverse isotropy） double-porosity medium is studied here to discuss the effect of different dip and azimuth angles of a fracture system. Based on the theories of fracture-induced HTI （horizental transverse isotropy） double-porosity medium, the softness and dispersion matrixes of fracture-induced TTI double-porosity medium are derived with the application of Bond transform, and finally, single-order velocity-stress equations are obtained. Furthermore, numerical simulations in xoz plane of 2.5 dimensional vector wavefield are carried out by the method of high-order staggered-grid finite-difference under perfect matched layer （PML） boundary conditions. The results show that the dipand azimuth angles of fractures have great impacts on seismic wave propagation, since the angles can cause the phenomena of shear wave splitting and, in the two-layer model of fracture-induced TTI double-porosity, converted shear wave splitting and shear wave sub-splitting. All of these increase the complexity of seismic wavefield and will lay a foundation of further studies on seismic wave propagation in actual earth layers.