Gong, F (Gong, Fei) ^{[1] [2]} ; Wang, GW (Wang, Guowei) ^[1] ; Kang, WJ (Kang, Wujiang) ^[3] ; Peng, SP (Peng, Suping) ^{[1] [2]} ; Zou, GG (Zou, Guangui) ^{[1] [2]} ; Zhang, ZJ (Zhang, Zhaoji) ^[1]

[1] College of Geoscience and Surveying Engineering, China University of Mining and Technology(Beijing), Beijing 100083, China.

[2] State Key Labortaory for Fine Exploration and Intelligent Development of Coal Resources, Beijing 100083, China.

[3] Research Institute of Exploration and Development, Liaohe Oilfield Company, Petrochina, Panjin Liaoning 124010, China.

Original publication

GONG Fei, WANG GuoWei, KANG WuJiang, PENG SuPing, ZOU GuanGui, ZHANG ZhaoJi. 2024. Study on elastic anisotropy and fracture characteristics of tectonic coals in Qinshui Basin. Chinese Journal of Geophysics (in Chinese), 67(9): 3544-3555, doi: 10.6038/cjg2024R0630.

http://www.geophy.cn//article/doi/10.6038/cjg2024R0630

Abstract

The elastic and anisotropy properties of coals are the basis for coalbed methane reservoir prediction, which are still poorly understood due to the complex mineral composition and pore structure. Therefore, four groups of middle-high order tectonic coals from the Qinshui Basin were selected to study their elastic and anisotropy properties, and ultrasonic tests under different confining pressure conditions were conducted in four directions: perpendicular to bedding planes, parallel to the face cleats, parallel to the butt cleats, and 45°to bedding planes. The results suggest that the velocities of the coals increase nonlinearly with the confining pressure, and the velocity pressure sensitivity obviously differs in different directions. The velocities of the coals display significant anisotropy, the velocity perpendicular to the butt cleat is the largest while that perpendicular to the bedding plane is the smallest. The velocity difference of coals along the face cleats and butt cleats directions is always less than 5%, which is much smaller than that between parallel and perpendicular to the bedding plane, suggesting that the coals can be approximately considered as transversely isotropic media. The velocity anisotropy parameters of the coals are negatively correlated with the confining pressure, and the variation caused by the confining pressure is much smaller than the velocity anisotropy at the highest confining pressure, indicating that the bedding plane is the dominant influence factor for elastic anisotropy of the coals, and the presence of fractures can enhance the anisotropy. Further, we quantitatively characterized the fracture density in different directions of the coals and discussed the relationship between fracture density and elastic properties and anisotropy. The results can provide rock physics support for the anisotropic inversion of coalbed methane reservoirs and the identification of face-butt cleats.

Keywords

Tectonic coal；Elastic properties；Anisotropy；Fracture