煤岩割理裂缝的发育导致其强度较低，钻井过程中钻井液的作用将会进一步增加井壁失稳的风险。采用室内实验、理论分析与数值模拟相结合的方法，揭示了钻井液作用下割理发育煤岩失稳机理：①煤岩黏土矿物主要由不易水化的高岭石组成，无易水化的蒙脱石存在且伊蒙混层的占比不高，因此水化膨胀（平均值为0.35%）和滚动分散性能（平均值为89.64%）较弱，煤岩失稳机制以力学作用为主。②垂直交错的面割理和端割理为钻井液侵入煤岩地层提供了流动通道，面割理的尺寸普遍大于端割理，因此，面割理更容易发生钻井液侵入造成井壁失稳。③钻井液侵入割理缝会导致井周地层压力上升，径向应力降低，增加井壁失稳的风险，其中对于高渗透率的面割理和穿过井眼的交叉割理，钻井液侵入更深，产生的井壁失稳风险更加严重。④割理缝的特征也会影响钻井液对煤岩割理的侵入深度。宽度更大、密度更高的割理缝中，钻井液侵入更深，并在井壁附近产生更高的压力，从而提高了井壁失稳的可能性。因此，应根据实际地层割理缝的尺寸大小设计钻井液随钻封堵粒子，且控制钻井液密度在合理的范围内，从而减少钻井液侵入引起井壁失稳情况的发生。研究提供了深入理解钻井液作用下割理发育煤岩失稳机理的新视角，为割理发育煤岩井壁稳定性分析提供了理论指导。

The development of cleat fractures in coal rocks leads to reduced strength, and the influence of drilling fluids during drilling further escalates the risk of borehole instability. By combining laboratory experiments, theoretical analysis, and numerical simulations, this study elucidates the mechanisms of instability in cleated coal rocks under the influence of drilling fluids: (1)the clay minerals in the coal rock are predominantly composed of kaolinite, which is resistant to hydration, with little to no montmorillonite present, and a low proportion of illite/smectite(I/S) mixed layers. Consequently, both hydration expansion (averaging 0.35%) and rolling dispersion (averaging 89.64%) are minimal, indicating the instability of coal rocks is primarily driven by mechanical factors. (2) Vertically intersecting face and end cleats create flow channels that allow drilling fluid to intrude into the coal strata. Since the dimensions of face cleats are generally larger than those of end cleats, face cleats are more susceptible to fluid intrusion, leading to borehole instability. (3) The intrusion of drilling fluid into the cleat fractures leads to an increase in formation pressure around the well and a reduction in radial stress, thereby raising the risk of borehole instability. Notably, for high-permeability face cleats and cross-cutting cleats that intersect the boreholes, deeper fluid intrusion further elevates formation pressure and diminishes radial stress, exacerbating the risk of instability. (4) Additionally, the characteristics of the cleats also affect the depth to which drilling fluid intrudes into the coal strata. In cleats with greater widths and higher densities, the drilling fluid penetrates deeper and generates higher pressures near the borehole wall, thereby increasing the likelihood of instability. Therefore, plugging particles should be added into the drilling fluid according to the size of the cleats and the density of the drilling fluid should be maintained within a reasonable range, in order to minimize borehole instability caused by fluid intrusion. The study provides a new perspective to understand the instability mechanism of cleats under the action of drilling fluid, and provides theoretical guidance for the analysis of coal rock wall stability.