Three‐dimensional multicomponent imaging of reservoir heterogeneity, Silo Field, Wyoming

Abstract

The Reservoir Characterization Project at Colorado School of Mines acquired a three‐dimensional (3-D) multicomponent survey over Silo field in southeastern Wyoming with the objective of imaging reservoir heterogeneity. A 3-D shear‐wave survey resolved spatial variations in the fracture distribution of Niobrara chalks by detecting small percentages of anisotropy induced by fractures in chalks of the Niobrara reservoir. In addition, the compressional‐wave survey imaged structural drape over a zone of deeper salt dissolution, which fractured the brittle chalks. Rotation analysis of the shear‐wave survey took advantage of its 3-D nature to identify an azimuthal pattern of anisotropy associated with vertical fractures, known as extensive dilatancy anisotropy (EDA). The shear‐wave data were sorted by shot‐to‐geophone azimuth to search for the orthorhombic pattern of anisotropy that might be expected from the combined effects of sedimentary layering and vertical fractures, but it was not found at Silo Field. Although groundroll contaminated some of the “pie slices” of azimuth in the rotation analysis, the redundancy of ten “pie slices” enabled us to determine an overall fracture orientation of N 58° W. The 3-D shear‐wave survey yielded a picture of fracture variability that could not be determined from well control alone. Fracture‐identification logs and production records were used in interpreting the anisotropy determined from the shear waves. Large positive values of anisotropy in the Niobrara interval, up to 5 percent, coincided with good producers in the field, while smaller magnitudes of anisotropy were tied to poor producers. Combining the multicomponent seismic recording with 3-D survey techniques added lateral resolution to the reservoir description and rendered a more complete understanding of the pattern of anisotropy resulting from fractures in the reservoir.

The fracture criticality of crustal rocks

“…(Such enhanced permeability is expected to vary with direction and be anisotropic, perhaps strongly anisotropic, which may mean that the behaviour needs to be closely controlled for the full benefits of enhanced permeability to be attained.) There have been several cases reported where the degree of shear-wave anisotropy have been correlated with hydrocarbon production (Cliet et al 1991;Li et al 1993), as well cases where higher production has been specifically associated with the presence of large fractures (Mueller 1991;Lewis et al 1991).…”

“…Hydraulic fracturing in the oil industry demonstrates that fluid-filled cracks are usually vertical and strike parallel to the maximum horizontal stress (or strictly, perpendicular to the minimum horizontal stress). In some cases, shear-wave splitting can be plausibly correlated with the presence of large fluid-filled fractures in sedimentary rocks, both directly (Mueller 1991;Lewis, Davis & Vuillermoz 1991), and indirectly (Cliet et al 1991;Li, Mueller & Crampin 1993). Shear-wave splitting in igneous and metamorphic rocks appears to be the result of cracks and microcracks with diameters less than a few centimetres, possibly much less (Holmes, Crampin & Young 1993).…”

Fractured reservoir delineation using multicomponent seismic data

“…Also, time-delays contain little information about fracture orientations, which must be deduced from other attributes. Squires et al (1989) and Lewis et al (1991) presented examples of interpreting interval time-delays from field data. Thomsen (1988) suggested that the differential amplitudes between the fast and slow split shear waves may be used to identify fracture zones in stacked seismic sections.…”

“…Multicomponent shear-wave reflection data have been acquired in a number of different areas for shear-wave studies (Alford 1986;Squires, Kim and Kim 1989;Lynn and Thomsen 1990). Threedimensional multicomponent reflection data have also been acquired for characterizing fractured reservoirs (Lewis, Davis and Vuillermoz 1991). Thus, evaluating the merits of the shear-wave information and establishing a consistent processing sequence for extracting the information are important for the development of shear-wave exploration.…”

A fast, robust method for detecting and characterizing azimuthal anisotropy with marinePSconverted waves, and its application to the west Svalbard continental slope

“…Elastic anisotropy is a common feature of the Earth's crust and upper mantle (Crampin & Lovell 1991). The anisotropic properties of these rocks are important to reservoir engineers and seismologists, amongst others, because an appropriate treatment of seismic data can reveal the orientation and intensity of subseismic fractures and stress regimes (Lewis et al 1991;Mueller 1991;Shuck et al 1996;Potters et al 1999). Evaluating and accounting for elastic anisotropy can also improve seismic imaging of the subsurface through more accurate velocity analyses (Verwest 1989;Ball 1995).…”