Experimental study on coal damage near wellbore under cyclic alternating load

Basis on the micro-fractures and cleats inside the coal, the damage is aggravated with the micro-fracture formation and propagation by the complex stress in the drilling and completion. A rock cyclic alternating stress experimental platform is built to load radial cyclic alternating stress load on coal samples with confining pressure and the crack propagation and coal damage are detected by CT scanning technology. The coal strain amplitude-time curve indicates that there is a critical value related to the cyclic stress parameters in the transformation from cyclic hardening to softening stage in the process of coal damage. SEM images of coal samples show that micro pores and fractures show large differences in size and shape and distinct heterogeneity in distribution. The experimental results show that the original fractures in coal are the decisive factors of the coal damage and crack size and distribution are the main factors affecting the crack propagation and evolution under cyclic alternating stress. Based on the dual pore structure, heterogeneity and multi-scale characteristics of coal cracks, the calculation model of coal damage variable under cyclic alternating stress is modified and the modified model is consistent with the CT scanning results.