关键词: 三维离散元, 煤层气, 水平井, 井壁稳定, 虚拟节理

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