Abstract:

Wellbore collapse is one of the main obstacles that prevent horizontal wells from the extensive use in coal seams.Coal rock mass has a great number of cleats and some fragmented zones,and is of low strength,so it is a great risk to drill in coal seams.Middle or high rank low-strength coal dominates in the Qinshui Basin,Shanxi Province.Hence,when water is used as a drilling fluid,it is easy for wellbore to collapse and the bottom hole assembly is readily buried underground.Especially for horizontal wells,the number of wells that collapsed has occupied about 32.65% of the total in the Qinshui district by January,2008.According to the concept of the discrete element method(DEM),problems of wellbore stabilities in coal beds can be attributed to a discontinuous problem.Coal rock mass is usually divided into small blocks by face cleats and butt cleats.Face cleats and butt cleats,on one hand,are regarded as boundaries between blocks,and on the other hand，act as a medium with which blocks interact.3DEC is a three-dimensional numerical program based on the discrete element method for discontinuum modeling.The present paper applied 3DEC to analyze horizontal wellbore stabilities in coal beds based on the core experimental and field-test data from the Qinshui Basin.Techniques of the virtual joint modeling and inside-outside two-layers modeling were used.The virtual joint modeling allowed a random generation of possible joints and a dynamic fracture analysis,and the inside-outside two-layers modeling could decrease amounts of computations and eliminated effects of boundaries.The actual distribution of both face and butt cleats in the No.3 coal seam of the Qinshui Basin was modeled by applying the above-mentioned modeling techniques.Influences of the strike of wellbores,hole diameter,bottom-hole pressure,and the density of cleats on wellbore stabilities were analyzed,respectively.The results demonstrated that when the strike of face cleats was perpendicular to that of wellbores,face cleats exerted hardly any influence on wellbore stabilities; when the strike of face cleats was parallel to that of wellbores,wellbores were the most unstable. Furthermore,the larger the hole diameter,the more unstable the wellbore; the larger the density of cleats,the poorer the stability of wellbores; and the higher the bottom-hole pressure,the more stable the wellbore was as seepage was excluded.

Key words: 3D discrete element method, coal-bed methane (CBM), horizontal well, wellbore stability, virtual joint

摘要：

为了阐明煤层中端割理和面割理等不连续面对井壁稳定的影响，利用三维离散元来分析煤层水平井井壁稳定，实现了在三维空间中对不连续面的仿真。在充分、真实反映煤岩割理特性的基础上，对煤层水平井进行了井壁稳定数值模拟分析。提出了虚拟节理和内外双层建模技术，虚拟节理技术可实现节理在各个方向的随机分布和动态的断裂分析，内外双层技术可降低计算量和消除边界的影响。模拟了沁水盆地3号煤层面割理和端割理的实际分布情况，最大程度反映了煤层的真实微观特征。通过分析井眼的走向、井径、井底压力和割理密度对井壁稳定的影响发现，当面割理的走向与井眼的走向相互垂直时，井壁最稳定；当两者相互平行时，井壁最不稳定。割理密度越大，井壁的稳定性越差；井径越大，井壁越不稳定。当不考虑渗流时，井底压力越大，井壁越稳定。

关键词: 三维离散元, 煤层气, 水平井, 井壁稳定, 虚拟节理