Chapter 9 - Horizontal Drilling in Coal Seams

Abstract

Vertical wells with hydrofracking reach a production limit at 3000–3500ft depth for loss of adequate permeability. The deeper coal seams where most of the coalbed methane reserve resides are only amenable to gas production by horizontal drilling from the surface and massive hydrofracking. Horizontal drilling had its beginning about 40 years ago for in-mine horizontal drilling for coal degasification. Because of the space limitations, the drill rig was small and consisted of five components: (1) the drill rig, (2) the auxiliary unit containing the power pack, (3) a drill cutting separation system so water could be reused, (4) a guidance system to guide the drill bit up, down, left or right, and (5) a downhole drill monitoring system that measured the pitch, roll, azimuth, and the distance of the drill bit from the roof of the coal seam. The data was digitized and sent to the surface by a hard wire or as an electromagnetic or acoustic signal. Each of these components is briefly described. Horizontal drilling from the surface requires a much bigger version of the small, permissible, in-mine drill rig. Typical commercial drill rigs and their range of operation are discussed. Drilling procedures both in the mine and on the surface are described. Steel casing schedules for different depth ranges are described. Water and sand schedules for (hydrofracking) a typical 5000ft long lateral are also described. Assuming two laterals (each 5000ft long) are drilled into a thick coal seam from the same location and both laterals are hydrofracked at 1000ft intervals, a production of 4–6MMCFD can be easily achieved. The process is expensive but if the price of gas is above $5/MCF, it can be highly profitable.