Coalbed Natural Gas Illustrations

Coalbed natural gas has been constantly venting gas. Sometimes gas will get into a water well. Gas is lighter than air and will pursue the path of least resistance to vent.

Operators are developing these three seams in the Tongue River area of Sheridan County, WY and Big Horn County, MT. Note the depth and thickness of the seams.

If you scraped away all the overburden and looked directly at the coal, the top illustration is a very simplified view of how the water and the gas are held in the coal seams. The coal is heavily fractured and very permeable. In the bottom illustration, the coal has been drilled into and water and gas have migrated from the fractures into the well bore. Once you lower the hydrostatic (water) pressure within the coal, it liberates the gas.

This is a diagram of how the coal is drilled to release the water and the gas. First, you drill down to the top of the coal, set the steel casing, pump cement down the steel casing and back up the exterior. This allows for complete isolation of the well bore from the more shallow coal formations. The cement is then drilled out and a drill is run to the bottom of the coal. Then a log is run to get an idea of the thickness of the coals and get a sample of the coal. From there, a submersible pump is run at the end of the tubing which is inside the steel casing. This pumps water up tubing and of to the applicable water management system. This lowers the hydrostatic pressure and liberates natural gas which flows up the annulus (the space between the casing and tubing). Then the gas is off to compression.