科研之友
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摘要
Fractures play an important role in most carbonate and unconventional reservoirs. The precise identification of fractures and their associated properties from seismic data have a significant impact on reservoir management and hydrocarbon recovery. Traditional seismic methods for fracture identification and characterization include shear birefringence and amplitude variations with offset and azimuth. These techniques are based on the equivalent medium theory, assuming that fracture dimensions and spacing are small relative to the seismic wavelength. Hence, the overall population of fractures is equivalent to a homogeneous anisotropic medium. We focus on discrete fracture sets, with spacing on the order of the seismic wavelength in which individual fractures have large areas but small apertures. Such fracture sets scatter seismic waves, and they are important to characterize because they affect fluid flow in the subsurface. Analyzing scattering-induced wave patterns observed in seismograms can provide crucial guidance for precise identification of fractures and their associated properties. We model seismic-wave propagation in fractured media following a published integral method with tetrahedral grid cells. This approach is derived from finite-element and finite-difference methods in heterogeneous media. It is flexible in modeling irregular interfaces and surface topography and has relatively low computational cost and memory requirements, which is essential for 3D simulations. The fractures are treated explicitly as interfaces with displacement discontinuities using the linear slip model. We implement the 3D explicit interface scheme using an irregular mesh. Our numerical examples demonstrate the effects of varying fracture height, spacing, and width on the seismic response. The wavefield scattering caused by the fractures is one of the key features. Accordingly, we establish a correlation between the scattering index and fracture spacing. Such an approach can be used to estimate fracture spacing with given fracture compliance and orientation.
- 出版日期2018-2
