Investigation of dynamic response of drilling parameters and deformation characteristics of coal around borehole during multi-stage reaming in tectonic coal

Tectonic coal refers to the coal with varying degrees of brittle fracture or ductile deformation under the action of one or multi-period tectonic stresses, showing the characteristics of low mechanical strength, weak cohesion and large deformation. Therefore, drilling in tectonic coal is easy to cause the aperture shrinkage or closure due to the poor anti-deformation capability. Enlarging the initial borehole can effectively increase the borehole diameter and reduce the risk of pipe jamming and burying without changing the borehole structure. In order to investigate the dynamic evolution characteristics of drilling parameters in the process of drilling and reaming in tectonic coal, a self-designed drilling experimental system that can monitor drilling parameters in real time was used in this study. Additionally, the acoustic emission (AE) response and borehole wall strain were observed to study the coal-breaking and the deformation characteristics of coal around borehole. The results demonstrate that the drilling rate and drilling torque increase sequentially in the initial drilling, primary reaming and secondary reaming stages. The magnitude of drilling rate is affected by both drilling thrust and rotational speed, whereas drilling torque is mainly affected by drilling thrust. The intensity of AE phenomena increases gradually at different stages of the borehole formation, which means that the drilling efficiency is improved and more strain energy is accumulated in the coal around the borehole during reaming. The borehole wall strain effectively reflects the convergence deformation of borehole wall, that is, the larger the borehole diameter, the greater the deformation of the borehole wall. The deformation of borehole is mainly affected by the redistribution of stress in the coal body, rather than the drilling parameters.