Seismic azimuthal amplitude variation with offset response analysis in coalbed methane reservoirs with aligned fractures using Biot theory

Abstract

The development of coalbed methane (CBM) relies on high-precision reservoir prediction and lithological inversion. Seismic amplitude variation with offset (AVO) technology is an important tool for fine-scale reservoir characterization. However, the seismic AVO response of CBM reservoirs is complex and is affected by seismic rock physics parameters at different scales. Microscopically, aligned fractures in CBM reservoirs produce complex anisotropy due to formation inclination. At the macroscopic scale, the thickness of CBM reservoirs within seismic frequency bands is comparable to the seismic wavelength and should therefore be treated as a layered medium. In addition, pore fluid significantly affects seismic wave propagation. Consequently, determining the azimuthal AVO response of CBM reservoirs in relation to seismic rock physics parameters at different scales can support high-precision reservoir prediction and lithological inversion. In this study, the primary–primary wave reflection coefficient for a two-phase layered medium was derived using Biot’s theory. Using this model, the response characteristics of the reflection coefficients with respect to seismic azimuth, aligned fracture parameters, reservoir thickness, and seismic main frequency were analyzed. A rotated staggered–grid finite–difference algorithm was employed to simulate wavefield characteristics separately for coal seams and surrounding strata. Seismic attributes were then used to characterize the seismic AVO response. The Z-direction seismic amplitude attributes and reflection coefficients showed similar trends in their responses to seismic rock-physical parameters. This study contributes to establishing a more precise seismic AVO response framework to support CBM reservoir prediction and high-quality lithological inversion.

Keywords

Azimuth / Amplitude variation with offset / Tilted transversely isotropic anisotropy / Fracture / Modeling

Funding

This study was supported by the National Natural Science Fund Projects (42274165,42474146), CNPC Innovation Fund (2024DQ02-0133), and the Fundamental Research Funds for the Central Universities (2024ZKPYDC03).

Conflict of interest

Fei Gong and Qiang Guo are Editorial Board Members of this journal, but were not in any way involved in the editorial and peer-review process conducted for this paper, directly or indirectly. The authors declare they have no competing interests.

Author contributions

Conceptualization: ZhaoJi Zhang, Fei Gong Formal analysis: ZhaoJi Zhang Investigation: ZhaoJi Zhang, Fei Gong Methodology: ZhaoJi Zhang, Hao Li Visualization: ZhaoJi Zhang, Hao Li Writing–original draft: ZhaoJi Zhang Writing–review & editing: GuanGui Zou, Qiang Guo, GuoWei Zhu

Availability of data

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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