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How Do You Get Coalbed Methane?

The Desorbtion of Methane

Understanding how the methane molecules are stored within the coal seam helps gas companies decide their production strategy. Generally, the more pressure exerted on a coal seam, the larger the number of methane molecules are attached to the coal. The process of molecular attachment is referred to as adsorption. In order to produce methane, we need to release, or desorb, the pressure on the coal. Since we can't change the geological depth of the coal, this usually occurs by removing the existing water that may be circulating in the coal seam. Water pressure in a coal seam is often referred to as hydrostatic pressure.

With the reduction of the pressure on the formation, methane desorbs from the coal and travels through the microporous coal matrix. Cleats, or natural fractures in the coal, provide pathways for the methane to travel to the wellbore and ultimately to the surface and into the pipeline. These fractures or cleats in the coal are essential as they provide permeability, or the ability of the gas to flow. Without a cleat system, the methane molecules have nowhere to flow, therefore without these 'roads' it becomes impossible to produce methane, regardless of the gas content within the coal seam.

Over time, as the methane molecules are desorbed from the coal as a result of the diminished pressure, water production will decrease and gas production will increase.

In most situations when CBM is produced the coal bearing formation is 'wet', meaning that the system, cleats, fractures and pore spaces are full of water. Alberta is unique in its geology because of the Horseshoe Canyon formation, where this cleat system is considered 'dry'. The term dry implies that that the cleats, fractures and pores are full of methane rather than water. In this circumstance, methane production begins as soon as the formation is drilled. There is no water to remove.

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