State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Southwest Petroleum University Cheng du ,China

针对煤储层强度低、割理又发育、钻井过程中井壁失稳严重的现象,采用现场取样统计分析、扫描电镜等试验方法,研究了该区煤岩宏微观结构及力学强度参数、变形破坏特征,并利用通用离散元数值模拟软件在充分反映煤岩割理结构基础上分析煤层气井井壁稳定性的影响因素。研究结果表明:该区煤岩割理发育,面割理和端割理近似垂直且多以网状和树枝状分布;煤岩力学参数离散性强,在单轴和三轴条件下煤岩应力应变曲线类似,都经历压密、弹性、屈服、脆性破坏4个阶段;在不考虑渗流作用时,井周最大剪切应力和剪切位移随井底压力变化规律一致,都呈先减小后增大趋势。井径越大,井周发生剪切位移越大,越容易失稳;井径一定时,割理密度越大,煤层井周块体越容易发生剪切滑落,井眼越不稳定,实现煤层气井安全、高效钻井。

In view of the low strength of coal seam, cleats development, in the process of drilling borehole wall instability phenomenon seriously,based on the filed sampled, statistical analysis and scanned electron microscopy, the macro and micro structure and mechanical strength characteristics and deformation failure characteristics of coal rock were studied. The general discrete element numerical simulation software was used to simulate the influence actors of wellbore stability in the process of CBM drilling based on the characteristics of coal cleats. The results showed:the cleats in coal rock were developed, the face cleats and the butt cleats were approximately vertical and mostly distributed in mesh and dendrite. The mechanical parameters of coal rock were discrete,The stress and strain curves of coal rock were similar under uniaxial and triaxial conditions, underwent several stages of compaction, elasticity, yielding and brittle fracture. Without considered the seepage action, the maximum shear stress and shear displacement of the borehole wall were consistent with the variation of the bottom hole pressure, and all decreased first and then increased.The bigger the well diameter, the greater the shear displacement at the well, the easier to lose stability.When the well diameter was constant, the greater the density of cleats, the more likely the coal bed blocks were to shear slip and the instability of the borehole. The results could provide certain reference for the safe and efficient coal seam gas drilling.