oa Efficient Production of Methane from Deeper Coal Seam

The primary problem with deep coal seams is decrease in permeability with depth. It is evident that hydraulic fracturing enhances the production significantly only when permeability is in the range of 1–10 md and minimal enhancement in production in 0.1–1 md permeability range but for permeabilities less than 0.1 md there is negligible enhancement in production.

This paper focuses on a revised permeability vs. depth curve which shows that the overly pessimistic exponential relation discussed earlier is incorrect due to the counterpoints like lower horizontal stress gradients in certain areas, unduly low permeability measured by slug tests because of skin effects from formation damage near the wellbore and the Kozeney-Carmen equation. This paper discusses the variation of permeability in a coal reservoir during production via three mechanisms: Klinkenberg effect and effective stress. The effect of variation of permeability on flow rates of methane is also discussed. At high stresses, the initial permeability reduction is significantly reduced and the increase in permeability due to decrease in gas pressure results in high flow rates during the latter part of the life of producing wells in deep coal beds.