What Controls CBM Production?
Most coals contain methane, but not all coals can produce methane economically. Successful CBM production depends upon several factors that vary between geological formations. These factors may include:
- Fracture Permeability and Gas Migration
In most cases naturally developed fracture networks within the coal are the most ideal situation for economic CBM development. Areas where geologic structures and localized faulting have occurred tend to induce natural fracturing, which increases the pathways for gas migration within the coal seam. Without fracture networks, the gas is trapped, or adsorbed, with the gas unable to flow. Additional fractures may be artificially created by forcing fluid or gas at high pressures into the coal seam. This process if often referred to as fracing.
- Coal maturation
The higher the coal rank, usually the greater its methane content.
- Geological structure
The depth and location of coal will influence the economics of its production
- Hydrostatic pressure
By reducing the water pressure in the coal seam, methane gas will desorb from the coal and migrate to the wellbore. This will only occur in coal seams that have water. It does not apply to the Horseshoe Canyon formation.
How is CBM Produced?
Depending upon the type of coal and fluid content in the formation, CBM wells may be drilled in several ways.
In the Horseshoe Canyon, where most of the CBM wells in Canada to date have been drilled, the coals are shallow. Because of its shallow depths, these wells are drilled vertically. That is a straight line is drilled down into the formation. In the Mannville and other deeper formations, the coal seams are much deeper and may be drilled vertically, horizontally or directionally. Directional and horizontal wells permit the operator to drill more than one well off the same lease pad, thereby reducing the amount of land and surface roads required.
Whether a well is drilled vertically or horizontally, they are often completed in the same manner. Typically, a steel-encased hole is drilled into the coal seam. The coal seam may then be fractured by injecting fluid (in Alberta this is normally liquefied nitrogen gas) down the well and into the seam's 'cleats' or natural fractures. If required, the coal seam is then dewatered. De-watering removes the water that naturally exists within the cleats of the coal using a pump located at the wellhead. Both fracing and de-watering reduces the pressure in the coal seam allowing the gas to flow into the wellbore and up to the surface. The gas is then collected and compressed into a pipeline to be shipped to your home.