Richard Van Dok

Azimuthal anisotropy in P-wave 3-D (multiazimuth) data

A naturally‐fractured gas reservoir in the Wind River Basin, Wyoming, is the site of a Department of Energy‐sponsored project to optimize seismic techniques for characterizing natural fractures (density, orientation). The target zone is the Lower Fort Union (LFU, depth 5400–10 000 ft, two‐way time ∼1.3–∼2 s), a low porosity, low permeability section in which fractures are necessary for commercial gas production. Flow rates incompatible with matrix porosity and permeability are observed.

Seismic anisotropy in microseismic event location analysis

Seismic velocity anisotropy has been known for many years to be an important factor in correctly imaging subsurface structures with reflection seismic methods (Alkhalifah et al., 1996). Fine vertical velocity layering naturally gives rise to VTI (vertical transverse isotropic) velocity in which seismic-wave velocity is faster in the horizontal direction than in the vertical direction. Similarly, vertically oriented natural fractures and stress fields influence seismic velocities in an azimuthal or compass direction which is described as HTI (horizontal transverse isotropic) velocity variation (Schoenberg and Sayers, 1995). HTI velocity analysis in seismic reflection imaging commonly has two main objectives: placing geologic features in the correct spatial location and characterizing fracture orientation and density to optimize oil and gas production strategy.

Near-surface shear-wave birefringence in the North Sea: Ekofisk 2D/4C test

Several recent studies have demonstrated the ability to measure the effects of shear-wave (S-wave) birefringence using mode-converted (PS) waves. Standard PS-wave processing relies on the assumption that the subsurface is horizontally isotropic. The two horizontal components are typically rotated (about the vertical axis) to a direction that is oriented radial and transverse to the source receiver geometry. In the presence of azimuthal anisotropy in the subsurface geology, the upcoming S-wave will split into two—one polarized in the fast direction (S1) and the other in the slow direction (S2). If S-wave birefringence is ignored, then the radial component will be comprised of a mixture of both fast and slow shear-wave energy. This mixing will result in a degradation of resolution and S/N ratio by destructive interference between the fast and slow S-wave arrivals.

Event registration and Vp/Vs correlation analysis in 4C processing

Accurately calibrating P-wave and PS-wave seismic sections is achievable using the currently available tools for event registration. This in an important step in wider use of multicomponent seismic data as it enables better interpretation and use of the combined data sets. It is also becoming more and more important in processing of multicomponent data as VTI anisotropy and Kirchhoff prestack time migration are included in the processing sequence. In this paper, we will look at the different tools for obtaining correct and accurate Vp/Vs ratios and discuss how including event registration in the processing sequence can improve the imaging as well as improve the quality of the Vp/Vs function.

Multicomponent 3‐D seismic pilot study in the Orinoco heavy oil belt

When the objectives of a 3‐D survey are very shallow, source and receiver line spacings have to be reduced in comparison to surveys designed for deeper targets. This increases the environmental impact and the cost per square kilometer.

Abstract: Detection of Naturally Fractured Tight Gas Reservoirs: Case Histories from the Uinta and Wind River Basins Using 2-D and 3-D Seismic Data

The goal of two U.S. DOE research projects directed by Blackhawk Geometries, contract no. DE-AC21-92MC28135 and DE-AC21-94MC31224, is the detection of gas-filled fractures using seismic methods. If highly fractured areas can be located using seismic techniques prior to drilling, it can greatly benefit field development. Shear wave seismic detects fractures, but it is expensive to acquire. If P-wave data can also detect fractures, the costs of finding fractured areas will be significantly reduced. In the Uinta Basin, 2 multi-component seismic lines and a multicomponent VSP were used to define fractures in the Upper Green River formation. The azimuthal difference of the P-wave AVO gradients showed open fracture directions consistent with the shear and VSP data. The dominant fracture direction at the Wind River Basin study area is East-West, and this information was used to design a processing sequence which splits the 3-D dataset into two subsets. Compressional waves which travel parallel to the dominant fracture direction will not sense the fractures, while compressional waves which travel perpendicular to the dominant fracture direction will sense the maximum influence of the fractures. The two data subsets were processed independently, and the azimuthally variant seismic attributes were compared to production from wells to calibrate the seismic attributes to production.