Summary
Seismic imaging has been very successful in the past several decades in finding structural traps for conventional reservoirs. However new imaging challenges have emerged for fractured and many unconventional reservoirs where the structural stratigraphy can be simple but extracting the internal fracture network information has become a first order issue for optimal hydrocarbon recovery.
By studying multiple scattering of seismic waves by locally periodic fractures, we have previously shown that, when the seismic acquisition has full azimuth, a double focusing Gaussian beams (DFGB) method is effective at determining fracture orientations for multiple fracture sets, the fracture spacing of each fracture set (even if the fractures are irregularly spaced), and the relative fracture compliance of each fracture set as a function of position in the reservoir. This information can be used to build reservoir fluid flow models for enhanced oil recovery.
For a strictly periodic fracture network (i.e., constant fracture spacing), the smallest spatial resolution that can be resolved is half of the wavelength. One can readily observe seismic wave scattering including multiple scattering if the wavelength is comparable to the constant fracture spacing. If the constant fracture spacing is very small compared to the wavelength, effective anisotropic medium theory can be used and no seismic wave scattering should be observed. However, if the fracture spacing is a random variable which follows, for instance, a power-law distribution typically observed at outcrops, even when the maximum fracture spacing is small compared to the wavelength, we can still observe strong scattering. This is because the random distribution of fractures tends to form fracture clusters, which introduce additional length scales such as the sizes of the clusters and distances between clusters that all are comparable to the seismic wavelength. In this case, the fracture spacing should be interpreted as the spacing between fracture clusters. Hence, our method can also be used to probe small fractures.
