基于Lagrange显式算法的井壁侧钻过程动态破碎规律

Dynamic crushing mechanism of well-wall in engraving stage of sidetracking based on Lagrange explicit algorithm

在煤层气多分支水平井裸眼悬空侧钻过程中,理解并掌握井眼连接段在地应力及钻具作用下的破碎及稳定机理对指导钻井过程尤为重要。结合Z1井岩芯样品的三轴抗压实验数据,运用Lagrange显式算法建立钻头-井壁-钻井液相互作用的瞬态显式有限元模型。采用DruckerPrager准则对裸眼悬空侧钻过程进行动态仿真模拟,研究钻头旋转冲击破岩过程中非线性动力作用及螺杆钻具接触对分支井眼形成过程的影响。将侧钻破岩至井眼形成阶段定义为"刻蚀过程",并细分为初、中、末3个阶段。通过大规模并行仿真计算,得到3个刻蚀阶段的井壁岩石的应力波动、破碎状态及刻蚀成井的钻压控制条件等参数;研究了"刻蚀过程"中井壁在地应力、钻具、钻井液等多种因素影响下的破碎规律,揭示了侧钻形成分支井眼过程中井壁的破碎机理。

During the process of hang sidetrack drilling in open hole of multilateral horizontal wells for extracting coalbed methane( CBM),it is very important to understand the mechanism of multilateral junctions' crushing and stabilizing under the effects of in-situ stress and drilling tools. This paper presented a dynamic explicit finite element model containing the interaction between the bit,well-bore and drilling fluid based on Lagrange explicit algorithm and triaxial compressive test data on core samples obtained from Well Z1. The dynamic process of hang sidetrack drilling in an open hole was simulated on the basis of Drucker-Prager criterion to investigate the influence of nonlinear and dynamic force from the rotary percussion of drill bit and the contact of drilling tools on the formation of branch well. The process from the start of sidetracking to the formation of branch well was defined as engraving stage which was further divided into three steps including 'early','middle'and 'last'stage. The fluctuating stress of wall rock,the way of crushing and the control condition of bit pressure for branch well's formation were obtained for these three stages. Furthermore,the paper comprehensively discusses the process and the mechanism of multilateral junctions' crushing in sidetracking under the effects of in-situ stress,drilling tools and fluid.