Borehole stability is of great concern in drilling wells in coal seams to extract coalbed methane, especially for horizontal wells within which excessive cuttings due to formation failure would easily incur the risk of stuck pipe. Due to the presence of cleats and natural fractures, the coal commonly features low mechanical strength, remarkable brittleness and heterogeneity, frequently resulting in borehole instability issues during drilling, completion and production. In this present work, we developed a hydro-mechanical model to predict the borehole breakout and assess the stability of wells drilled in coal seams. The deformation and damage of the coal and the pore fluid flow within the coal are treated in a fully coupled manner. The mechanical behavior of the coal is represented by using an elastoplastic model incorporating post-peak strain softening, which has been evidenced to be essential for faithfully predicting the shape and size of the damage zone. In addition, the permeability of the coal within the damage zone is related to the damage state of the coal through an empirical relation. An inertia-based stabilizing method is proposed and implemented to overcome the loss of numerical convergence due to the abrupt strain softening constitutive behavior. The model has been applied to simulate the borehole breakout evolution and analyze the stability of coalbed methane wells drilled in the Qinshui basin located in north China. The effects of the drilling mud density as well as the mud infiltration are demonstrated, and some implications for drilling stable wells in coal seams have been provided.

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