Analytical Model of Wellbore Stability of Fractured Coal Seam Considering the Effect of Cleat Filler and Analysis of Influencing Factors

Abstract

Currently, coal borehole collapses frequently occur during drilling. Considering that the coal near to the wellbore is cut into blocks, and the cleat filler of the coal influences the stress distribution near the wellbore, a new theoretical solution of a near-wellbore Stress Field in coal bed wells is established. In addition, according to the limit equilibrium theory and the E.MG-C criterion, the limit sliding formula of the quadrilateral and triangular block is deduced, and the slipping direction of the blocks is further judged. Finally, the wellbore stability model of the coal seam is established. The accuracy of the theoretical model is verified through a numerical method by using the PFC software. Based upon this wellbore stability theoretical model of coal, many cleat affecting factors such as cleat spacing, cleat length, cleat angle and the cleat geometric position, are studied, and the results show that a quadrilateral block slides off more easily than a triangular block under the same boundary condition; the bigger the cleat spacing and cleat length are, the lower is the risk that blocks slide off, and increasing the cleat angle could cause blocks to slide off easily. Under the same boundary condition, whether blocks slide off or not is closely related to the well round angle. study how the size-effect of a horizontal wellbore in a coal seam impacts on wellbore stability by RFPA 2D . Chu and Shen et al. [12][13][14] proposed fracture mechanics and a 3-D discrete element method to analyze the mechanical properties of the coal bed methane wellbore stability and the method to determine the window of drilling fluid density. Since the coal seam was non-continuum, Chen et al. [15] established the discrete element model of the collapse pressure of a coal seam based on non-continuum mechanics considering mud intrusion strata. Zhu et al. [16] proposed the model of coal seam wellbore stability based on the discrete element method. Qu et al. [17] presented the evaluation model of borehole stability in a coal seam by using a stress intensity factor to describe the concentrative degree of crack stress. Zhao and Zhang et al. [18,19] concluded the stress field expression around the broken coal seam wellbore by considering the cleat as the crack and computing the additional stress fields of cracks. But those papers did not distinguish the differences between the cracks and cleats of coals and rocks [20]. Cracks are fractures which can easily separate from each other, and cleats are endogenetic fractures, and this means that they contain fillers in the fracture gap space. Through the researches, the impact of the cleat filler and its connected state on the stress field around the entire wellbore were critical [21]. Through reviewing related literatures and materials, cleats are divided into full filled cleats, filled cleats and unfilled cleats [22]. Therefore, the impact on the near stress field of the cleat filler around the entire wellbore could not be eliminated.According to the current literature review, it was ext...

Impact of Nitrogen Injection on Pore Structure and Adsorption Capacity of High Volatility Bituminous Coal

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Dewatering Optimization of Coal Seam Gas Production Wells: A Review and Future Perspectives

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