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.
Highlights
It is an essential part of the unconventional development of energy to exploit coalbed methane inChina [1]
Specialists have achieved a number of results [7,8,9,10] on the study of the wellbore stability of oil and gas wells, which can give us ideas on solving the problems of a broken coal seam
The boundary condition (B.C.) of the numerical model is the same as that of the analytical the quadrilateral block, the only way to slide is by overcoming the strength in both sides along the sliding, and breaking the limit equilibrium in the side which is perpendicular to the sliding direction
Summary
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 Those papers did not distinguish the differences between the cracks and cleats of coals and rocks [20]. The stability model of the broken coal seam wellbore is established, which evaluates the block sliding near the wellbore, by introducing the E.MG-C criterion and the limit equilibrium method to analyze the strength failure and tensile resistance of the cleats. The sliding condition of the triquetrous block and the quadrilateral block are taken into account, and the accuracy of the analytical model is verified through a numerical method by the Particle Flow Code (PFC) software
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