What Is Coal Bed Methane?
Coal bed methane is a natural gas by-product of coal formation. During coal formation, organic matter is chemically broken down into simple organic compounds. Methane is a by-product of this breakdown.
Coal is very porous but lacks matrix permeability. In other words, water can seep into coal but can't flow through it. Naturally occurring fractures in coal allow ground water to permeate the coal and provide the means through which the methane is stored in the coal bed. Due to coal's porous nature, methane gas produced during coal formation is absorbed into the coal bed and held in place by the weight of the surrounding groundwater.
How Is Coal Bed Methane Extracted?
In order to commercially produce coal bed methane, it is necessary to decrease the water pressure within the coal bed. When the pressure is decreased, the methane gas naturally desorbs from the coal and migrates through the coal bed.
This natural phenomenon is the basis for coal bed methane production. In extracting methane from coal beds, a well is drilled down to the coal bed and the sides of the well are then encased in concrete. A water pump is dropped down into the coal bed and the top of the well is sealed with a venting pipe to collect the methane. Large amounts of groundwater are pumped out of the coal bed, causing a corresponding decrease in water pressure. The decreased water pressure allows the methane to escape from the coal and migrate along the coal fractures and up into the well. The methane is then pumped from the well through the venting pipe where it is compressed and sold.
Over the past 2 decades in the Powder River Basin in Northeastern Wyoming and elsewhere in the Rocky Mountain Basins from Central Canada to Mexico there has been a surge of drilling for coal bed methane (CBM) gas. There are literally thousands of wells drilled and in production as well as thousands that will be drilled in the future decades.
One major component of a CBM well is the presence of an aquifer that submerges coal seams. While the presence of water is a major factor in the creation of the methane molecules, typically there are large amounts of water present, and the shear weight of the water creates too much hydro-static pressure, thus preventing methane gas bubbles from traveling up the well bore.
The solution to the pressure problem creates a second problem for producers. Current production methods require wells to be pumped free of water to allow the liberation of the methane gas from the coal seams. Pumping, transfer, and disposal of the water bears a high cost.
CBM aquifers have a high salt absorption rate (SAR) which renders water from the wells harmful to the environment making disposal difficult.
Producers have handled this problem by pumping the water to storage pits for evaporation. This practice is not economically feasible as regulations presently in effect significantly limit this method of water disposal. Proposed legislation would prohibit it completely.
The practice of pumping the water out of CBM the wells creates another problem, it appears to create rapid gas flow at first but it actually KILLS the microbe culture that CREATES the gas - which, of course, begins the industry accepted rapid decline curve in gas production in CBM wells.