Feasibility of Seismic Characterization of Multiple Fracture Sets
Estimation of parameters of multiple fracture sets is often required for successful exploration and development of naturally fractured reservoirs. The goal of this paper is to determine the maximum number of fracture sets of a certain rheological type which, in principle, can be resolved from seismic data. The main underlying assumption is that an estimate of the complete effective stiffness tensor has been obtained, for example, from multiazimuth, multicomponent surface seismic and vertical seismic profiling (VSP) data. Although typically only a subset of the stiffness elements (or some of their combinations) may be available, this study helps to establish the limits of seismic fracture-detection algorithms. The number of uniquely resolvable fracture systems depends on the anisotropy of the host rock and the rheology and orientation of the fractures. Somewhat surprisingly, it is possible to characterize fewer vertical fracture sets than dipping ones, even though in the latter case the fracture dip has to be found from the data. For the simplest, rotationally invariant fractures embedded in either isotropic or transversely isotropic with a vertical symmetry axis (VTI) host rock, the stiffness tensor can be inverted for up to two vertical or four dipping fracture sets. In contrast, only one fracture set of the most general (microcorrugated) type, regardless of its orientation, is constrained by the effective stiffnesses. These results can be used to guide the development of seismic fracture-characterization algorithms that should address important practical issues of data acquisition, processing, and inversion for particular fracture models.
Seismic characterization of multiple fracture sets from multicomponent , multiazimuth , 3 D data : Rulison Field , CO
Conventional fracture-characterization methods assume the presence of a single set of aligned, vertical cracks in the subsurface. We relax this assumption and demonstrate the feasibility of seismic characterization of multiple fracture sets. Our technique relies on recent theoretical findings indicating that multiple, differently oriented, possibly intersecting planar cracks embedded in an otherwise isotropic host rock result in an approximately orthotropic effective media. Here, the governing parameters of crack-induced orthotropy are estimated from 3D, wide-azimuth, multicomponent (P-, fast shear-, and slow shear-wave) seismic reflection data acquired over the tight-gas Rulison Field in Colorado. We use strong azimuthal variations of the normal-moveout velocities to invert for the interval crack densities, fracture orientations, type of fluid infill, and velocities of Pand S-waves in the unfractured rock. Our inversion procedure identifies a set of cracks approximately aligned in the WNW-ESE direction in the western part of the study area and multiple, likely intersecting fractures in its eastern part. We validate both our underlying theoretical model and the obtained estimates by a number of independent measurements: (i) the estimated fluid-infill parameter indicates dry cracks as expected for the gas-producing sandstones at Rulison; (ii) our model predicts almost elliptical anisotropy and, indeed, the long-spread P-wave data exhibit no nonhyperbolic moveout; (iii) the obtained crack orientations are supported by well observations. As a by-product of fracture characterization, we build an anisotropic velocity model of the Rulison reservoir which, we believe, is the first example of an orthotropic velocity model constructed from field surface seismic data.
Azimuthal variation in AVO response for fractured gas sands
Natural fractures in reservoirs play an important role in determining fluid flow during production, and hence the density and orientation of fractures is of great interest. In the presence of aligned vertical fractures, the reflection amplitude at finite offset varies with azimuth. The effect of natural fractures on the azimuthal AVO response from a gas‐sandstone reservoir encased within shale is investigated. A simple expression for the difference in P‐wave reflection coefficient from the top of the reservoir parallel and perpendicular to the strike of the fractures is obtained in terms of the normal and tangential compliances, ZN and ZT, of the fractures. This expression is valid for small anisotropy and material contrasts and is compared with the results of numerical modelling. For a given value of ZT, the azimuthal variation in reflection coefficient at moderate offsets is found to increase with decreasing ZN/ZT. For gas‐filled open fractures ZN/ZT ≈ 1, but a lower ratio of ZN/ZT may result from the presence of cement or clay within the fractures, or from the presence of a fluid with non‐zero bulk modulus. For ZN/ZT = 1 and moderate offsets, the variation with offset of the reflection coefficient from the top of the fractured unit is dominated by the contrast in Poisson's ratio between the gas sand and the overlying shale, the effect of fractures only becoming noticeable as the critical angle for the unfractured sandstone is approached. However, for reflections from the base of the fractured unit, the variation in reflection amplitude with azimuth is much greater at conventional seismic offsets than for the reflection from the top. Azimuthal variations in the strength of the reflection from the top of the reservoir depend only on the variation in reflection coefficient, whereas the raypath is also a function of azimuth for reflections from the base of the fractured unit, leading to stronger, more visible, variations of AVO with azimuth. It follows that an azimuthal variation in AVO due to fractures in the overburden may be misinterpreted as due to the presence of aligned fractures in the reservoir.