Chih-Ping Lu

The construction of subsurface illumination and amplitude maps via ray tracing

Editors note: This article was selected as the Best Poster Paper at SEGs 1999 Annual Meeting. Due to its quantitative nature, the 1999 Best Student Poster Paper will not be published in TLE but in a future issue of Gℯℴ𝓅𝒽𝓎𝓈𝒾𝒸𝓈, without undergoing peer-review. The goal of a seismic survey is to illuminate subsurface geologic formations that may hold hydrocarbon accumulations. Conventional seismic survey design relies on the assumption that uniform midpoint coverage will lead to uniform illumination in the subsurface as long as each midpoint is hit by a sufficient range of offsets. In areas of complex velocity structure, severe wavefield distortions lead to irregular subsurface illumination patterns, even if surface midpoint maps show a uniform distribution. A more appropriate approach is to design seismic surveys to ensure illumination of key subsurface horizons. The difference between midpoint coverages and subsurface illumination patterns is particularly large in salt-prone areas (Muerdter et al., 1997). Due to severe wave distortion through complex, high-velocity salt bodies, conventional design methods that result in relatively uniform surface coverage (Figure 1) generate uneven amplitudes and shadow zones on subsalt horizons, an effect that is shown clearly by ray-trace modeling of an entire seismic survey (Figure 2). Figure 1. Total hits in each surface bin resulting from a 3-D seismic survey collected along east-west lines. Distances are in kilofeet. Figure 2. Amplitude of a subsurface …

Mitigation of overburden effects in fracture prediction using azimuthal AVO analysis: An example from a Middle East carbonate field

The ability to identify fracture clusters and corridors and their prevalent direction within many carbonates and unconventional shale gas/tight gas reservoirs may have a significant impact on field development planning as well as on the placement of individual wells. We believe seismic fracture prediction provides the best opportunity to identify the spatial distribution of fracture corridors, but the reliability of seismic fracture detection technology is constantly being questioned. The criticism results from the degree to which the acquisition footprint, random and coherent noise in the seismic data, and near-surface/overburden issues affect extracted seismic “fracture” attributes. Therefore, a key issue is the separation of artifacts caused by the acquisition footprint and near-surface or overburden anisotropy/structural variations from the anomalies caused by the presence of fractures.

Seismic fracture prediction using azimuthal AVO analysis in a Middle East carbonate field: workflow and mitigation of overburden effects

Summary Seismic-fracture prediction is based on the fact that, in fractured rocks, seismic velocity varies with the direction of wave propagation relative to the fracture orientation (azimuthal anisotropy), which causes seismic amplitude to vary with azimuth (azimuthal AVO or AVOAZ). The theory of wave propagation in fractured rocks is well documented, but many issues remain to be resolved before this technology can be used routinely. These issues include overcoming problems related to acquisition and overburden effects and determining optimal data processing and signal enhancement methods. . In this paper, we apply a targetoriented AVOAZ processing workflow, which includes, but is not limited to, data conditioning, suppression of the acquisition footprint and mitigation of near-surface and overburden effects to extract fracture-anisotropy attributes in a carbonate oil field in UAE.

Study Geophysical Response of Middle East Carbonate Reservoir using Computational Rock Physics Approach

Summary Development of carbonate rock physics model is difficult because pore systems are more complex in carbonate than they are in clastics. The best way to describe pore structure is through 3D µCT image of rock pore space. Instead of traditional effective medium based modeling by taking assumption of pore geometry, we adopt computational rock physics approach in this study. Multi-resolution µCT images are taken for carbonate cores belonging to different facies from middle east carbonate reservoir. Electrical conductivity and elastic properties (Vp, Vs) are computed on 3D rock micro-tomographis using finite difference (FD) and finite element method (FEM). To further extend predicting capability, a family of 3D model granular porous media with different porosity, pore (grain) aspect ratio, pore (grain) size distribution, pore connectivity and spatial arrangement are built to represent different carbonate petrophysical pore types. Modeling results compare well with core, log measurements. Direct link between rock microstructure and its elastic, electrical behavior is built up using computational rock physics. Finally, AVO forward modeling is built to quantify porosity, fluid saturation and lithology (facies) effect on seismic response for middle east carbonate reservoir.

A field study of azimuthal seismic anisotropy in fractured carbonates at Canyon Lake, central Texas

Many important hydrocarbon fields worldwide produce from fractured carbonate reservoirs. For those fields, azimuthal seismic anisotropy potentially provides a means to map fracture orientation and distribution in the subsurface.

P-Wave Seismic Anisotropy In a Fractured Carbonate Reservoir: A Case Study From EastTexas

Summary We present a case study where we calculated P-wave seismic anisotropy for a land, 3D seismic survey at a gas field in east Texas. We calculated the azimuthal AVO (AzAVO) and azimuthal velocity (AzNMO) anisotropy. We compared the results with interpreted faults, fractures from oriented core, image logs, present day stress, and a dipole sonic log. High- anisotropy anomalies align with faults on the flanks of the structure. Low seismic anisotropy characterizes the crest of the structure, and may be an artifact of an associated amplitude shadow. Locally, orientations predicted from seismic anisotropy agree with our subsurface fracture observations. Full quantitative fracture interpretation is restricted, however, by signal noise, local reduction in inversion quality by an amplitude artifact, and limited subsurface fracture correlation.

Azimuthal AVO Workflow And Evaluation At an East Texas Fractured Reservoir

Summary We applied azimuthal anisotropy of amplitude versus offset (AzAVO) inversion technique to land 3D seismic data at an east Texas gas field. In general, AzAVO provides geologically interpretable anisotropy anomalies that correlate with previously interpreted faults and also correlate with the few subsurface data (FMI and cross-dipole sonic) that we have. The inversion quality is affected by signal/noise in the data and the application of random noise attenuation in the pre-processing enhances the results. We are encouraged to have developed an integrated workflow for the processing and interpretation of azimuthal seismic anisotropy.