Amplitude variation with offset and azimuth inversion to predict and evaluate coal seam fracture parameters

doi:10.1007/s11707-022-1017-y

Front. Earth Sci.

2023, Vol. 17

Issue (2) : 505-513 https://doi.org/10.1007/s11707-022-1017-y

RESEARCH ARTICLE

Amplitude variation with offset and azimuth inversion to predict and evaluate coal seam fracture parameters

Haibo WU^1,²(

), Shujie ZHU³, Qinjie LIU², Shouhua DONG⁴, Yanhui HUANG^1,⁵, Pingsong ZHANG¹

¹. School of Earth and Environment, Anhui University of Science and Technology, Huainan 232001, China
². Institute of Energy, Hefei Comprehensive National Science Center (Anhui Energy Laboratory), Hefei 230051, China
³. Xi’an Research Institute, China Coal Technology & Engineering Group, Xi’an 710076, China
⁴. School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, China
⁵. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China

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Abstract

Amplitude variation with offset and azimuth (AVOA) inversion is a mainstream method for predicting and evaluating fracture parameters of conventional oil and gas reservoirs. However, its application to coal seams is limited because of the specificity of the equivalent media model for coal—also, the traditional seismic acquisition system employed in coal fields falls within a narrow azimuth. In this study, we initially derived a P‒P wave reflection coefficient approximation formula for coal seams, which is directly expressed in terms of fracture parameters using the Schoenberg linear-slide model and Hudson model. We analyzed the P‒P wave reflection coefficient’s response to the fracture parameters using a two-layer forward model. Accordingly, we designed a two-step inversion workflow for AVOA inversion of the fracture parameters. Thereafter, high-density wide-azimuth pre-stack 3D seismic data were utilized for inverting the fracture density and strike of the target coal seam. The inversion accuracy was constrained by Student’s t-distribution testing. The analysis and validation of the inversion results revealed that the relative fracture density corresponds to fault locations, with the strike of the fractures and faults mainly at 0°. Therefore, the AVOA inversion method and technical workflow proposed here can be used to efficiently predict and evaluate fracture parameters of coal seams.

Keywords equivalent media model fracture density and strike azimuth Student’s t-distribution

Corresponding Author(s): Haibo WU

Online First Date: 07 June 2023 Issue Date: 04 August 2023

Cite this article:

Haibo WU,Shujie ZHU,Qinjie LIU, et al. Amplitude variation with offset and azimuth inversion to predict and evaluate coal seam fracture parameters[J]. Front. Earth Sci., 2023, 17(2): 505-513.

URL:

https://academic.hep.com.cn/fesci/EN/10.1007/s11707-022-1017-y
https://academic.hep.com.cn/fesci/EN/Y2023/V17/I2/505

Fig.1 Two-layer forward model with a horizontal transverse isotropic (HTI) fracture coal seam below the reflection surface.

Fig.2 P–P wave reflection coefficient responses to the fracture density at different incidence angles and azimuths. (a) θ is equal to 0°; (b) θ is equal to 15°; (c) θ is equal to 30°; (d) θ is equal to 45°.

Fig.3 Technical workflow involving the amplitude variation with offset and azimuth (AVOA) inversion for coal seam fracture parameters.

Fig.4 Location, measure point distribution, and acquisition system parameters for the surveyed area.

Fig.5 Lithologic column of the surveyed area. No. 6 coal seam representing the target is at the top of the Taiyuan Formation (C2t).

Fig.6 Common-depth-point (CDP) gather displaying reflection events attributed to the target coal seam.

Fig.7 Plot illustrating ellipse fitting of the gradient term. The cyan dots represent the value of the gradient term at different azimuths, and the red curve represents the fitting result.

Fig.8 Display of inversion results for the relative fracture density (

Δ e

) and fracture strike (

β

). The colors denote the relative fracture density, while the short black lines represent the fracture strike.

Fig.9 Rose diagram displaying the fracture strike, with the numbers in black denoting the azimuth, and those in blue representing the fracture strike.

Fig.10 Plot comparing the fracture parameter inversion and fault interpretation results.

Fig.11 Rose diagram displaying the fault strike data for the surveyed area. The numbers in black denote the azimuths, while those in blue represent the fault strike results.

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[1]	CHENG Jiubing, WANG Huazhong, GENG Jianhua, MA Zaitian. Methods and applications of 3-D wave equation common-azimuth prestack migration[J]. Front. Earth Sci., 2007, 1(3): 366-372.

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