A large number of studies have shown that there are numerous cleavage planes and cracks in coal rock, and these weak planes are the main factors affecting the mechanical strength of coal rock. In order to quantitatively study the mechanism, rock samples were soaked in three kinds of fluids, namely "clean water", "original drilling fluid" and "new drilling fluid with strong plugging ability" and then these soaked samples and dry rock sample were respectively tested by high-temperature and high-pressure triaxial mechanical tester. The failure and deformation law of coal rock were quantitatively described, and the influence of confining pressure, cleavage plane and fluid on the mechanical parameters (cohesion, internal friction angle, elastic modulus, Poisson's ratio) was analyzed. A model for predicting the collapse pressure of coal seam considering the effects of seepage and multiple cleavage planes was established. The mechanical parameters of dry rock samples and the rock samples soaked in original drilling fluid and new drilling fluid were introduced into the model to quantitatively explain the impact of fluid invasion on coal rock formation collapse.
The Jurassic strata in the northern Kuqa area of the Tarim Basin are deeply buried, and there are multiple sets of coal seams in the J2kz-J1y layers, with a burial depth of about 3600 m~4800 m1,2 ( Wang, D.2018; Mao J,1999) Well collapse, leakage and diameter expansion often occur, which seriously affect drilling speed and cementing quality in the later stage, and restrict oil and gas production. Studies have shown that during the diagenesis of coal, a large number of cleats will be formed in the coalification stage3,4(Jian, B,2001; Cao, 2021), and these cleats will reduce rock strength. The drilling fluid intrudes into the cleat in the drilling process due to pressure difference, thus playing a lubricating role to a certain extent5 (Chen, 2013), which further reduces the mechanical strength of the coal matrix block and cleat 6,7,8 ( Wang, 2020; Li, 2022; Han, 2022) and is even more unfavorable for wellbore stability9 ( Wang, 2010).