COAL BED METHANE
Q &A Format
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.
What Are the Environmental Effects of Coal Bed Methane Extraction?
One of the environmental effects of coal bed methane extraction is the immense quantity of water pumped out of the coal bed aquifers. On average, approximately 12-15 gallons of water per minute are pumped from each well. During the initial phase of production, water is pumped at a very high rate. The extracted water is typically discharged into local streams or reinjected into the ground. Where the coal bed groundwater is relatively pure, surface discharge is the most common method of disposal. Smaller quantities are sometime stored in large pits for evaporation but this method is inefficient to deal with massive quantities of extracted groundwater. The removal and disposal of so much groundwater raises several concerns.
One concern is that drainage of a coal bed aquifer will cause shallower aquifers to drain into the cavity created by the coal bed water extraction. This is a particular concern for local landowners relying on well water pumped from shallow aquifers, which is often the case, as aquifers used for domestic water wells tend to be shallower than coal bed aquifers. In several reported cases, local water wells have gone dry after coal bed methane operations have begun.
A similar concern exists for coal bed aquifers that are tributaries to surface waters or adjacent groundwater aquifers, i.e., coal bed aquifers that contribute to other water sources. The drainage of tributary coal bed aquifers can cause a corresponding decline in the water levels of the contributory water sources. Consequently, water depletion from coal bed methane operations can have a significant impact on residents, farmers and businesses relying on affected water supplies.
Surface Water Discharge.
Because surface discharge is the most common disposal option for the extracted coal bed water,
the compositional characteristics of coal bed water can have a tremendous impact on the surrounding ecology. The quality of coal bed water varies considerably from well to well and basin to basin, but, on average, the deeper the coal bed, the more saline the water becomes. Other compositional elements typically seen in extracted coal bed water include:
Major Cations (positively charged ions such as sodium, potassium, magnesium, & calcium)
Major Anions (negatively charged ions such as chlorine, sulfate, & hydrogen carbonate)
Trace Elements & Metals (iron, manganese, barium, chromium, arsenic, selenium, & mercury)
Organics (hydrocarbons and additives).
The saline and sodic quality of coal bed water can have catastrophic impacts on local agriculture when discharged into local waterways. The moderate to high levels of salt in coal bed water can destroy soils and decrease crop production. The salts gather in the root base of plants, making it harder for the plants to extract water from the soil and inhibiting growth. As many farmers make use of stream and river diversion to water their crops and grazing fields, the surface discharge of coal bed water can negatively impact local agriculture.
Unlike salinity, which measures the quantity of dissolved salts in water, sodic water is measured by the proportion of sodium to calcium and magnesium. Sodic water interacts with fine soils, like clay, and results in the formation of a hard crust that severely impairs water and air permeation. Sodic water can cause a sharp decrease in the growth of crops and other vegetation.
Chemical & Radioactive Contamination.
Because coal bed methane extraction depends upon the natural fractures within the coal bed, gas companies routinely attempt to increase the extent of coal bed fracturing in order to boost methane production. Hydraulic fracturing is a technique used for this purpose. Hydraulic fracturing pumps a mixture of heavy chemicals, water, sand and/or other materials down an extraction well under extremely high pressure in order to achieve the desired fracturing. Hydraulic fracturing raises serious concerns because of the chemicals being used and their impact on the local ecology.
If the coal bed aquifer is tributary to surface water or other groundwater aquifers, chemical contamination can spread into domestic, agricultural, and industrial water supplies. Because hydraulic fracturing typically precedes the water extraction phase, much of the fracturing fluid will be pumped out of the aquifer along with the bulk of the groundwater. Where surface discharge is used to dispose of the extracted groundwater, the fracturing fluid is discharged along with the groundwater directly into local waterways, potentially contaminating water sources relied upon by local communities.
The types of chemicals used in fracturing fluids vary from company to company. In some states, companies are not required to disclose the chemicals used in their mining operations, so the extent of contamination is still unknown. Based on Material Safety Data Sheets obtained from several coal bed methane operators, many of the chemicals used are highly toxic, water soluble, volatile, and highly mobile-some are even radioactive.
Despite the fact that large amounts of hazardous chemicals are known to be injected directly into the coal bed aquifer, there is shockingly little oversight. Coal bed aquifers often contain potable or high-quality water and the injection of fracturing fluids into such water sources can permanently contaminate a viable source of water.
Coal bed methane production is a rapidly growing industry that will undoubted continue to expand under the nation’s demand for alternative energy sources. Although global climate concerns have illustrated the need for the development of clean burning fossil fuels, coal bed methane production is not without its environmental hazards. Water depletion from coal bed methane production can adversely impact adjacent residents, farmers and businesses that rely on local groundwater and surface waters. The saline and sodic quality of coal bed water can inhibit plant growth when discharged into local waterways. Chemical contamination resulting from hydraulic fracturing poses a significant threat to domestic, agricultural, and industrial water supplies and anyone unfortunate enough to be exposed to such hazardous materials. While the use of cleaner energy sources must be encouraged, such development should not proceed to the detriment of local communities.