Coalbed cleat porosity is the natural fracture system within coal seams, which plays a crucial role in how gases (like methane) and fluids are stored and move through the coal. Imagine coal as a layered, organic rock, it’s not just a solid block but has a network of tiny cracks called "cleats." These cleats form during the coalification process, where plant material transforms into coal under heat and pressure, and they create pathways and spaces within the coal structure.
There are typically two main types of cleats: face cleats (the primary, more continuous fractures) and butt cleats (shorter, secondary fractures that often connect the face cleats). The porosity, essentially the percentage of void space in the coal, comes from these cleats rather than from the coal matrix itself, which is usually quite dense and has low inherent porosity. This cleat system is what makes coalbed methane extraction possible, as the gas is mostly adsorbed onto the coal’s internal surfaces and can flow through these fractures when pressure is reduced, like during mining or drilling.
In practical terms, cleat porosity affects how much gas a coal seam can hold and how easily it can be tapped. It varies depending on factors like the coal’s rank (how mature it is), depth, and local geological stresses. Typically, cleat porosity ranges from about 1% to 5% of the coal’s volume, though it’s often on the lower end compared to conventional reservoirs like sandstone.
