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ABSTRACT 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.
P-Wave Seismic Anisotropy In a Fractured Carbonate Reservoir: A Case Study From EastTexas
Johns, Mary K. (ExxonMobil Upstream Research) | Wang, David Y. (ExxonMobil Upstream Research) | Sun, Sam Z. (ExxonMobil Upstream Research) | Lu, Chih-Ping (ExxonMobil Upstream Research) | Xu, Shiyu (ExxonMobil Upstream Research) | Susewind, Ken (ExxonMobil Production Company) | Zhou, Da (ExxonMobil Production Company)
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. Introduction A network of open, connected fractures adds porosity to and increases the permeability of many reservoirs worldwide. Seismic anisotropy is a fracture detection technique most sensitive to the open fractures that are important for flow. Theoretical models show that both travel time and amplitude attributes vary with azimuth in rocks with vertically aligned fractures Azimuthal AVO (AzAVO) detects anisotropy from the amplitude variation of a particular event (PP or PS). Azimuthal NMO (AzNMO) detects anisotropy using the normal moveout travel time attribute of a prestack CDP gather. Previous seismic models have predicted that shear wave data may have a clearer anisotropic response than P-wave data (e.g. Lynn and Thomsen, 1990; Li, 1997). In this study, however, we considered only P-wave anisotropy. Because of the wide availability of P-wave seismic data, we expect that the development of P-wave anisotropic techniques to have a broad business impact. For this study, our objectives were 1) to develop AzNMO and AzAVO workflows for fracture detection, 2) apply the workflows for a land survey with typical land P-wave acquisition and data quality, and 3) integrate the seismic anisotropy results with seismic-scale structures, stacked data attribute volumes, dipole sonic logs, fractures in core and image logs, stress indicators, and production data. We applied AzAVO and AzNMO techniques to help characterize the fractures for a multi-reservoir gas field in east Texas. One of its reservoirs is a 140 ft thick wackestone, with 2-18% porosity and 0.01-0.2 md permeability. Its low matrix permeability, together with core fracture observations, suggest that fractures assist gas flow in the reservoir. Fractures may have formed due to a combination of folding and faulting. Local salt withdrawal gently folded the field into a broad dome. The fold is faulted at its crest by northeast-trending normal faults. Fractures from the two oriented cores also strike northeast; however, core fractures are typically filled with calcite-cement. Drilling induced fractures strike east-northeast, indicating the maximum horizontal stress direction. Additionally, a reprocessed dipole sonic log displays an east-northeast fast velocity orientation. In general, the quality of fracture prediction from seismic anisotropy depends on several factors: geology, acquisition, processing, and calibration. For this study, the chief uncertainties in AzAVO were caused by the amplitude shadow. Low signal/noise, incomplete azimuthal distribution, and low fold may have also affected the seismic anisotropy calculations.
- Geology > Petroleum Play Type > Unconventional Play > Fractured Carbonate Reservoir Play (0.41)
- Geology > Geological Subdiscipline > Geomechanics (0.35)
- Geology > Structural Geology > Fault (0.35)
Stress-induced Velocity Anisotropy of Unconsolidated Sand Under Realistic Reservoir Stress Conditions
Chen, Ganglin (ExxonMobil Upstream Research Co.) | Yale, David (ExxonMobil Upstream Research Co.) | Huang, Xiaojun (ExxonMobil Upstream Research Co.) | Xu, Shiyu (ExxonMobil Upstream Research Co.) | Finn, Chris (ExxonMobil Upstream Research Co., now at ExxonMobil Development Co.) | Boitnott, Greg (New England Research)
ABSTRACT Ultrasonic velocity measurements were made on dry and oil saturated samples/cores of unconsolidated sands to investigate the stress-induced velocity anisotropy under realistic reservoir stress conditions. Instrumentation was arranged to simultaneously measure five velocities (axial P, axial S, radial P, radial S polarized radially, and radial S polarized axially) and the axial and radial deformation of the samples in a single run. Within the experimental uncertainties, the measurements show: (1) Stress-induced velocity anisotropy in unconsolidated sands could be a major contributor to the azimuthal shear wave anisotropy observed in sonic logs from some of the West Africa wells; (2) Stress-induced velocity anisotropy is stress-path dependent; (3) P-wave stress-induced anisotropy is stronger than S-wave stress-induced anisotropy; (4) Vp/Vs ratio could increase or decrease with increasing stress; (5) For dry samples, P-wave velocity is related to the stress component in the direction of wave propagation, whereas S-wave velocity is related to the average of the stress components in the directions of wave propagation and particle motion. However, large uncertainties still exist in the exact amount of stress-induced velocity anisotropy. More measurement data are needed for better reservoir characterization where stress regimes are non-hydrostatic.
- Africa > West Africa (0.25)
- North America > United States (0.16)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Mineral (0.96)
- Geophysics > Seismic Surveying > Seismic Processing (0.50)
- Geophysics > Seismic Surveying > Seismic Interpretation (0.30)
Summary Multi-component seismic data offer not only seismic reflections of pure compressional wave (PP) but also of converted shear waves (PSV and PSH) resulting from conversions at stratum interfaces in the subsurface. Different types of waves have different responses to subsurface geology. The reflection amplitude responses of different types of waves have been investigated and compared through a numerical modeling study. The shot-receiver offset and azimuthal angle have been varied relative to the subsurface anisotropy symmetry direction. Our results show that PSV data are more sensitive to azimuthal anisotropy than PP data. This makes the converted wave a better tool for subsurface azimuthal anisotropy (or fracture) detection. The study also demonstrates that the higher-order term in PP-wave amplitude versus offset (AVO) expression (beyond the traditional intercept and slope terms) can be important for fracture detection. Feasibility modeling study before field application is essential to better understand the relative importance of each term in the azimuthal AVO response and the appropriate incidence angle range for azimuthal anisotropy detections. Introduction Multi-component seismic technology has proved useful in imaging targets below gas chimneys, and imaging reservoir rocks with low P-wave impedance contrast but high shear impedance contrast relative to overburden rocks and in lithology discrimination. The motivation of this study is to explore the potential benefits of multi-component seismic data in fracture detection. The focus of this study is to compare the sensitivity to azimuthal anisotropy between P-wave to P-wave (PP) reflection and P-wave to SV-wave (PSV) reflection under ideal conditions without considering the effects of noise and tuning. Through this comparison study, we want to see what type of wave, which parameters, and what incident angle range are better for azimuthal anisotropy detection. A Transversely Isotropic model with a Vertical symmetry axis (VTI) is a good description for horizontally layered sediments, especially shale formations. A Horizontally Transverse Isotropy (HTI) model is usually used to describe fractured rock with a system of parallel vertical fractures. The seismic wave reflection from a VTI/HTI interface provides an ideal basic model for studying the responses of reflection seismic amplitude with azimuthal anisotropy. Earlier theoretical work (Vavrycuk, 1998 and 1999; Ruger, 1997 and 1998) on such a model has laid the foundation for fracture detection using PP waves. More recent work (Vavrycuk, 1999; Cherepanov, 2004) on a PSV reflection from a VTI/HTI interface has laid a framework of fracture detection using PS waves in parallel to that of PP waves. In our modeling study, we use both of those analytical approximations and exact solutions of the reflection amplitudes to perform azimuthal AVO analyses and comparisons based on the simple VTI/HTI interface model. Method The PP and PSv reflection amplitudes are normally approximated as serial expansions in terms of the incidence angle (or offset). The azimuthal dependence of the reflection amplitude is included in the expansion coefficients of the incidence angle. The PP reflection from a VTI/HTI interface under the condition of small rock property contrast and small incidence angle is approximated as (Vavrycuk, 1998 and 1999; Ruger, 1997 and 1998),
- Geology > Geological Subdiscipline (0.69)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.54)