煤层气勘探开发过程中井壁稳定性问题突出，传统连续介质力学模型难以准确评价井眼失效机理。在分析破碎性煤层物理性质的基础上，以沁水盆地南部沁端区块3号煤层为例，将虚拟煤岩体定义为基质和裂隙两个部分，采用Monte Carlo方法生成了随机的裂隙网络，通过通用离散元程序UDEC进行了破碎性煤层段井壁稳定数值模拟分析，研究了不同钻井方式条件下煤层井壁稳定性。结果表明：随机裂隙的存在使得井周塑形范围加大，井眼更易垮塌失稳。当井筒压力较小时，井壁主要发生拉伸破坏；当井筒压力较大时，井壁主要发生剪切破坏。在该煤层参数条件下煤层井眼的最佳稳定井筒压力区间值为6~7 MPa。%In the process of drilling in broken coal seam, the borehole instability-related problem is serious. Tradi-tional mechanical model is difficult to evaluate the failure mechunism of borehole. In this paper, based on the characteristics of broken coal seam, a case study was conducted in 3# coal seam of the Qingduan block, Qinshui basin, the coal and the rock was defined as two parts: matrix and fissure. Monte Carlo method was used to generate the random fissure network. The effect of drilling methods on wellbore stability was numerically simulated by the universal discrete element code (UDEC). The results show that micro underbalanced and micro balanced drilling can be adopted in coal drilling. Tensile failure occurs at low wellbore pressure; coal body is gradually sheared un-der big wellbore pressure, the best stable wellbore pressure interval value is 6~7 MPa in this case.
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