Abstract A wellbore strengthening model for CBM drilling was analytically established based on the fracture mechanics, considering the cleat system. In this model, the stress field induced by the multi-cleats opened was firstly solved based on the fracture mechanics and the principle of the linear superposition. Especially, a failure criterion was introduced to judge whether the cleat was opening or closed. Then, the total stress was obtained by superimposing the stresses induced by the drilling fluid pressure, the formation pressure, the in-situ stress and the opened cleat. More importantly, the cleat-tip stress intensity factors before and after strengthening were calculated and compared to elaborate the validity of the wellbore strengthening. The results show that the cleat-tip stress intensity factors decreases obviously after the wellbore strengthening. Meanwhile, the analytical model shows an agreement to the finite element results for the stress distribution and the cleat-tip stress intensity factor. Finally, the parametric analysis was performed to reveal that: the smaller distance from the location of LCMs to the wellbore wall and the higher sealing efficiency of LCMs are positive for the wellbore strengthening; the larger number of cleats and the longer length of the opened cleat are harmful to the wellbore strengthening. This research has a contribution on establishing the wellbore strengthening model with multi-cleats.

中文翻译：

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考虑割理系统复杂分布的井筒加固分析模型

摘要 基于断裂力学，考虑割理系统，分析建立了煤层气钻井井筒加固模型。在该模型中，首先基于断裂力学和线性叠加原理求解了多条割理张开引起的应力场。特别是引入了失效准则来判断夹板是打开还是关闭。然后，将钻井液压力、地层压力、地应力和割理张开引起的应力叠加得到总应力。更重要的是，计算并比较了加固前后的割理尖端应力强度因子，阐述了井筒加固的有效性。结果表明，井筒加固后割理尖端应力强度因子明显降低。同时，分析模型表明应力分布和割理尖端应力强度因子与有限元结果一致。最后通过参数分析表明：LCMs位置到井壁距离越小，LCMs密封效率越高，对井筒加固有积极作用；割理数量越多，打开的割理长度越长，不利于井筒加固。该研究对建立多割理井筒加固模型有一定的贡献。分析模型表明应力分布和割理尖端应力强度因子的有限元结果一致。最后通过参数分析表明：LCMs位置到井壁距离越小，LCMs密封效率越高，对井筒加固有积极作用；割理数量越多，打开的割理长度越长，不利于井筒加固。该研究对建立多割理井筒加固模型有一定的贡献。分析模型表明应力分布和割理尖端应力强度因子的有限元结果一致。最后通过参数分析表明：LCMs位置到井壁距离越小，LCMs密封效率越高，对井筒加固有积极作用；割理数量越多，打开的割理长度越长，不利于井筒加固。该研究对建立多割理井筒加固模型有一定的贡献。割理数量越多，打开的割理长度越长，不利于井筒加固。该研究对建立多割理井筒加固模型有一定的贡献。割理数量越多，打开的割理长度越长，不利于井筒加固。该研究对建立多割理井筒加固模型有一定的贡献。