Research on coal stress measurement and assessment while drilling method based on multiple drilling parameters in large diameter drilling

ZHAO Tongbin; ZHAO Zhigang; QI Yanshan; YIN Yanchun; NIU Xu; LI Qifan

doi:10.12438/cst.2024-1465

Abstract：Since high coal stress is a necessary condition for the occurrence of rock bursts,timely and accurate measurement and assess-ment of coal stress are critical for rock burst early warning systems.Large diameter drilling is the most commonly employed method for pre-pressure relief,hazard mitigation,and stress control in rock burst prevention.The development and application of coal stress measure-ment-while-drilling(MSWD)technology based on large diameter pressure relief boreholes can provide substantial coal stress data from underground coal mines.Based on the coal body cutting and failure characteristics,a mechanical analysis of bit-induced coal breakage was conducted.The relationship between bit force and coal stress was studied using the bit cutting force as a link,and a mechanical model for coal body drilling was established.An inversion equation for coal body stress,incorporating multiple drilling parameters,was proposed.The accuracy of the proposed model was verified through indoor drilling experiments.A measurement-while-drilling device,specifically designed for large diameter pressure relief drilling rigs,was developed and used for field measurements in underground coal mines.Data on bit displacement,rotation speed,and torque over time were obtained,and coal stress at different drilling depths was calculated.The stress distribution pattern in the pressure relief borehole area was presented,and a comparative analysis was conducted between the drilling energy,drilling time,and coal stress data.The results show that the average error between the loading stress from indoor drilling experi-ments and the stress calculated from the mechanical model is 4.9%,with the maximum error being 15.7%.The field-measured coal stress increases first and then decreases with hole depth,which is consistent with the distribution pattern of abutment pressure in the coal wall.The curves of coal stress,drilling time,and drilling energy with hole depth in the same borehole are nearly identical,indicating that drilling in high-stress zones consumes more energy and requires longer drilling time.By comparing the stress of adjacent boreholes,it was found that the first drilled borehole exhibited a higher peak stress,while the second drilled borehole showed a lower peak,with the peak shifting deeper,indicating a significant pressure relief effect in the first drilled borehole.By establishing a quantitative relationship between drilling parameters and coal stress,this study enables the in-situ measurement of coal stress based on large diameter pressure relief boreholes,ex-tends the use of large diameter pressure relief boreholes from engineering applications to stress measurement applications,realizing the in-tegration of coal pressure relief with stress measurement and evaluation.