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Title: Factors facilitating or limiting the use of AVO for coal-bed methane

Abstract

There are similarities and differences in employing amplitude variation with offset (AVO) to explore for gas-sand reservoirs, as opposed to coal-bed methane (CBM) reservoirs. The main similarity is that large Poisson's ratio contrasts, resulting in AVO gradient anomalies, are expected for both kinds of reservoirs. The main difference is that cleating and fracturing raise the Poisson's ratio of a coal seam as it improves its reservoir potential for CBM, while gas always lowers the Poisson's ratio of a sandstone reservoir. The top of gas sands usually has a negative AVO gradient, leading to a class one, two, or three anomaly depending on the impedance contrast with the overlying caprock. On the other hand, the top of a CBM reservoir has a positive AVO gradient, leading to a class four anomaly. Three environmental factors may limit the usage of AVO for CBM reservoirs: the smaller contrast in Poisson's ratio between a CBM reservoir and its surrounding rock, variations in the caprock of a specific CBM reservoir, and the fact that CBM is not always free to collect at structurally high points in the reservoir. However, other factors work in favor of using AVO. The strikingly high reflection amplitude of coal improvesmore » signal/noise ratio and hence the reliability of AVO measurements. The relatively simple characteristics of AVO anomalies make them easy to interpret. Because faults are known to improve the quality of CBM reservoirs, faults accompanied by AVO anomalies would be especially convincing. A 3D-AVO example offered in this paper shows that AVO might be helpful to delineate methane-rich sweet spots within coal seams.« less

Authors:
; ; ; ;  [1]
  1. China University of Mining & Technology, Beijing (China)
Publication Date:
OSTI Identifier:
20813048
Resource Type:
Journal Article
Journal Name:
Geophysics
Additional Journal Information:
Journal Volume: 71; Journal Issue: 4; Journal ID: ISSN 0016-8033
Country of Publication:
United States
Language:
English
Subject:
01 COAL, LIGNITE, AND PEAT; METHANE; COAL DEPOSITS; COAL SEAMS; EXPLORATION; MAPPING; RESERVOIR ROCK; AMPLITUDES; GEOLOGIC FAULTS; SEISMIC WAVES

Citation Formats

Peng, S P, Chen, H J, Yang, R Z, Gao, Y F, and Chen, X P. Factors facilitating or limiting the use of AVO for coal-bed methane. United States: N. p., 2006. Web. doi:10.1190/1.2217137.
Peng, S P, Chen, H J, Yang, R Z, Gao, Y F, & Chen, X P. Factors facilitating or limiting the use of AVO for coal-bed methane. United States. https://doi.org/10.1190/1.2217137
Peng, S P, Chen, H J, Yang, R Z, Gao, Y F, and Chen, X P. 2006. "Factors facilitating or limiting the use of AVO for coal-bed methane". United States. https://doi.org/10.1190/1.2217137.
@article{osti_20813048,
title = {Factors facilitating or limiting the use of AVO for coal-bed methane},
author = {Peng, S P and Chen, H J and Yang, R Z and Gao, Y F and Chen, X P},
abstractNote = {There are similarities and differences in employing amplitude variation with offset (AVO) to explore for gas-sand reservoirs, as opposed to coal-bed methane (CBM) reservoirs. The main similarity is that large Poisson's ratio contrasts, resulting in AVO gradient anomalies, are expected for both kinds of reservoirs. The main difference is that cleating and fracturing raise the Poisson's ratio of a coal seam as it improves its reservoir potential for CBM, while gas always lowers the Poisson's ratio of a sandstone reservoir. The top of gas sands usually has a negative AVO gradient, leading to a class one, two, or three anomaly depending on the impedance contrast with the overlying caprock. On the other hand, the top of a CBM reservoir has a positive AVO gradient, leading to a class four anomaly. Three environmental factors may limit the usage of AVO for CBM reservoirs: the smaller contrast in Poisson's ratio between a CBM reservoir and its surrounding rock, variations in the caprock of a specific CBM reservoir, and the fact that CBM is not always free to collect at structurally high points in the reservoir. However, other factors work in favor of using AVO. The strikingly high reflection amplitude of coal improves signal/noise ratio and hence the reliability of AVO measurements. The relatively simple characteristics of AVO anomalies make them easy to interpret. Because faults are known to improve the quality of CBM reservoirs, faults accompanied by AVO anomalies would be especially convincing. A 3D-AVO example offered in this paper shows that AVO might be helpful to delineate methane-rich sweet spots within coal seams.},
doi = {10.1190/1.2217137},
url = {https://www.osti.gov/biblio/20813048}, journal = {Geophysics},
issn = {0016-8033},
number = 4,
volume = 71,
place = {United States},
year = {Sat Jul 15 00:00:00 EDT 2006},
month = {Sat Jul 15 00:00:00 EDT 2006}
}