In the 2006 SME conference proceedings is a paper, Installation of a state-of-the-art ventilation shaft at a New Mexico coal mine by S. Bessinger of the San Juan Coal Company, T. Palm of Mencon, and A. Zeni of Shaft Drillers, Inc. Most of the paper deals with successful installation of the new shaft in difficult conditions. An interesting case history in itself. But the part I reproduce here describes the need for a new ventilation shaft (I edit for brevity.)

San Juan Coal Company operates a coal mine near Farmington, New Mexico. The coal seam is prone to spontaneous combustion. A bleederless ventilation system is used on the longwall. In expanding to a new phase it was determined that longer panels (up to 12,500 ft) would be used. Due to methane and hydrogen sulfide a wrap-around ventilation system could not be used—ventilation pressures would have had to be increased beyond conventional limits. The company decided that a small ventilation shaft in the back (longwall set up area) of the extension would draw adequate air past the longwall seals to maintain low gas levels and permit inspections to be undertaken. Modeling established that the shaft would have to handle air volumes from 40,000 cfm to 185,000 cfm. It was established that a 350-ft deep, 5-ft diameter shaft would be needed. The shaft was installed using blind drilling.

Like most you will probably not be able to access the paper, but more information is available from this link. More on the technicalities of bleederless ventilation systems is available at this link.