Research on the Hydration Damage Mechanism and Wellbore Stability of Deep Fractured Coal-Rock

ABSTRACT: As China's oil and gas sector shifts its research focus toward clean energy transition, coalbed methane (CBM) exploration and development are emerging as a critical frontier. Coal reservoirs, as typical unconventional reservoirs, exhibit well-developed fracture networks that significantly increase the risk of drilling fluid filtrate invasion during operations, making drilling fluid a key factor influencing the mechanical strength of coal wellbores.This study focuses on deep fractured coal seams in the Junggar Basin. Integrating scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses, we investigated the microstructural composition of coal and systematically characterized the hydration mechanisms of drilling fluids. By testing the moisture absorption behavior and uniaxial/triaxial mechanical damage characteristics of coal under optimized ratios of inhibitive and plugging additives, a wellbore stability evaluation model incorporating hydration effects was established, with collapse cycles quantitatively defined.Key findings reveal that fluid loss through coal fractures and hydration of clay minerals synergistically degrade coal strength. The optimized drilling fluid demonstrated exceptional performance:Plugging efficiency improved by 37%;Swelling rate reduced to 0.97%;Cuttings recovery rate increased to 91.6%;Permeability restoration enhanced by 9.3%.Notably, the enhanced plugging and inhibition capabilities extended the wellbore collapse cycle by 12.3%.This research provides both theoretical and practical guidance for drilling fluid design optimization, effectively improving wellbore stability and reservoir protection in CBM exploration.