Numerical modeling of gas flow in deformed well casing for the prediction of local resistance coefficients pertinent to longwall mining and its engineering evaluation

During gob gas venthole (GGV) drainage, the borehole casing is to withstand various forces induced by the movement of overlying strata of the longwall panel so as to become deformed. Tensile deformation, compressive deformation and shear deformation are the most typical deformation modes of the casing. Once deformation of the casing occurs, the flow resistance of gas in casing increases sharply, which causes the volumetric flow of gob gas recovery to decrease. Through an analysis of the cross-sectional shapes of three typical casing deformations, the characteristic parameters of each kind of deformation were determined. In addition, a numerical simulation of the gas flow field was carried out using the four casing models, i.e., one without deformation and three with typical deformations. The changing characteristics of the local resistance coefficient of the deformed casing with varying characteristic parameters of corresponding deformation were analyzed. In addition, this study compared the local resistance coefficient of casings for the three deformation models and deduced that the local resistance coefficient resulting from shear deformation is the largest. Furthermore, by considering the flow field characteristics of gas in the three models, this study determined that the vortex induced by the separation of the boundary layer in the areas of deformation was a major cause of local resistance. Finally, a method for predicting the characteristic parameter values of casing deformation was proposed, and a case study was performed. The research results provide a theoretical basis for predicting the influence of casing deformation on GGV drainage.