Experimental Study on Coal Damage Near Wellbore Under Cyclic Alternating Load
Authors: Bi Yansen , Gao Deli , Xian Baoan 
Published: September 2022

ABSTRACT:
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.

1. INTRODUCTION
The micro cracks in coal are expanded under the cyclic alternating load acting on the borehole during drilling and completion, which aggravates the coal damage of wellbore. The wellbore instability and collapse caused by coal damage may lead to blocking, sticking accidents and even wellbore scrap. In other words, cleats and other natural fractures in coal seams may be activated by cyclic alternating stress to improve the permeability (Barton et al., 1995; Finkbeiner et al., 1997; Wiprut and Zoback, 2000; Nazaralizadeh and Rasouli, 2011). Some scholars (Chen et al., 2012; Zhou et al., 2010; Yang et al., 2019) studied the characteristics of rock deformation, strength, energy evolution, acoustic emission signal and damage under cyclic alternating stress by rock mechanics experiment (Li et al., 2019; Yang et al., 2020; Wang et al., 2010) and found the residual strain, plastic hysteresis loop, fatigue threshold, stress memory effect, Kaiser Effect and Felicity Effect of rock (Momeni et al., 2015; Duan et al., 2020). Feng (Feng et al., 2020) studied the rock properties variation with cumulative damage by carrying out true triaxle cyclic loading and unloading tests on granite, marble and sandstone. Duan (Duan et al., 2021)analyzed the influence of true triaxle graded cyclic intermediate principal stress on coal deformation, energy consumption and damage through experiments of coal mechanics and damage characteristics under triaxle cyclic loading. CT detection device was fixed on triaxle loading test equipment to detect rock micro damage under cyclic loading in real time (Ge et al., 2021). Krajcinovic (Krajcinovic et al., 1982) proposed a rock damage constitutive model based on statistical damage variables according to the random distribution characteristics of rock internal defects.