Abstract: A method for gasifying subterranean coal deposits by positioning a cased injection well to extend from the surface into the coal deposit with the injection well extending horizontally through the lower portion of the coal deposit with the horizontal portion of the well being cased with a perforated casing; positioning an injection tubing in the injection well; positioning a production well to extend from the surface to a point near the lower end of the injection well; igniting the coal deposit; gasifying a portion of the coal deposit between the bottom of the production well and the lower end of the injection tubing well by injecting a free-oxygen containing gas into the coal deposit through the injection tubing and recovering product gases through the production well and thereafter gasifying a second portion of the coal deposit by withdrawing the injection tubing a selected distance and thereafter injecting free-oxygen containing gas into the coal deposit and recovering product gases from the production well

Abstract: A shear sub for installation in the drill string used to rotate a drill bit in the drilling of wells includes a mandrel extending upwardly for connection to the drill string. A housing slidably enclosing the mandrel extends downwardly below the mandrel and is threaded for connection directly, or indirectly, through joints of drill pipe to the drill bit. Rotation and longitudinal movement of the housing relative to the mandrel during drilling is prevented by shear pins which engage the housing and the mandrel to hold the housing in a position in which lugs extending inwardly therefrom are directly above and in alignment with splines extending outwardly from the mandrel. If an obstruction to drilling occurs that results in excessive torque on the drill string, the shear pins are sheared. On lifting and rotating the drill string, the mandrel moves upwardly relative to the housing whereupon the lugs interlock with the splines to allow rotation of the drill bit as it is lifted off bottom.