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What is coal bed methane?A component of natural gas is a substance called methane (CH4). Coalbed methane - CBM - is the same compound as the methane which is the primary energy source in natural gas. Coalbed methane, however, is found in coal seams, and produced by non-traditional means, and therefore while it is sold and used the same as traditional natural gas, its production is very different. Coalbed methane is generated either from a biological process as a result of microbial action or from a thermal process as a result of increasing heat with depth of the coal. Often a coal seam is saturated with water and methane is held in the coal by the pressure of the water.Currently, natural gas from coalbeds is approximately 7% of the total natural gas production in the United States, nearly one-third of the estimated 184 trillion cubic feet (TCF) of gas in the Rocky Mountain Region is in coal seams. An estimated 200 tcf of potentially recoverable CBM resources lie in the Cook Inlet basin. (John Tanigawa, Evergreen Resources, Inc. 2003).
How do gas companies extract the methane from the coal seam?Methane travels with the ground water being pumped from the coal from a well drilled and completed in a coal seam that contains methane and fixed with a water pump. Since methane has very low solubility in water it separates from the water in the well before the water enters the pump. The extraction of CBM involves pumping the available water from the coal seam aquifer in order to reduce the water pressure that holds the gas in the coal seam. CBM producers try not to dewater the coal seam. The goal is to decrease the water pressure (or head of water) in the coal seam to just above the top of the seam. However, sometimes the water level drops into the coal seam. Water moving from the coal seam to the well bore encourages gas migration toward the well. As this water pressure is released, the gas will rise and is separated from the water and piped away.
How do they estimate the amount of methane gas that will come from a region underlain by coal?There are two popular methods for estimating recoverable methane gas from a coal seam. One method requires estimating methane reserves by boring a hole through the coal seam, extracting a core from the bore hole, then trapping the amount of methane from the coal core. If a number of cores are drilled and methane gas release is observed, one can get a rough idea of the amount of gas available in a region. The limitations to this method are: 1) there is much disturbance to the coal seam core before gas release is finally measured; 2) it is expensive and 3) not every region of potential CBM development has been drilled and explored.Another method of estimating the amount of methane in a region underlain by coal is through a series of calculations based on information we already know about the coal in the region and the feasibility of CBM development. For instance, the Montana Bureau of Mines and Geology estimated the amount of recoverable CBM in the Powder River Basin using the following information:
The Environmental Impact Statement for CBM development in the Powder River Basin estimated the amount of coal in the region based on the reported tonnage of coal in the region multiplied by 50 ft3 of methane per ton of coal, irregardless of seam thickness, depth or proximity to the outcrop.
What are the current management practices with the CBM produced water?Currently, outside Alaska, CBM product water is managed by the following methods:
Another option being proposed for disposal of CBM product water in eastern Wyoming and Montana, and now in Alaska, is to reinject the CBM produced water back into an aquifer(s). This practice is happening in the southwest U.S. where CBM produced water is injected into deeper formations below CBM-bearing coal. This approach avoids surface discharge. Many opinions exist, and the feasibility - economic, physical, and environmental - of either reinjecting CBM produced water to the coal seam from which it was pumped or injecting it into an aquifer above or below the CBM-bearing coal seam is being investigated.
Why are people concerned about CBM produced water?There are several concerns about coal bed methane development and how to manage the water which is pumped to extract the methane.The quantity of the CBM produced water: The extraction of CBM involves pumping tremendous volumes of water from the saturated coal seam in order to release the water pressure holding the gas in the coal seam. What to do with the huge volume of often marginal-quality CBM produced water is a source of much debate. Each well is expected to produce 5 to 20 gallons of water per minute. At 12 gallons per minute, a total of 17,280 gallons of water per day may come from one well. It is common to have to have one well every 80 acres. In the Powder River Basin, there are up to three methane-bearing coal seams; therefore, there may be three wells per 80 acres. The quality of CBM produced water and its effect on soil and plants: Coalbed methane product water has a modestly high salinity hazard and often a very high sodium hazard based on standards used for irrigation suitability. Irrigation with water of CBM produced water quality on range or crop lands can be very risky. With time, salts from the produced water will accumulate in the root zone to concentrations which will affect plant growth. Saline conditions stunt plant growth because plants must work harder to extract water from the soil. The sodium hazard of CBM produced water poses additional threats to certain soil resources. Sodium poses a more troublesome problem in soils with more than 30% clay. On such soils, sodium changes soil physical properties, leading to poor drainage and crusting. Irrigation with sodic water on sandy soils does not cause crusting and poor drainage as the sodium is more readily leached from the soil profile. Current research at Montana State University shows water with sodium levels equal to that in typical CBM produced water will permanently degrade the physical and chemical properties of heavier, clay soils, making such soils completely unsuitable for plant growth. Montana State University recommends irrigation with CBM produced water only on the coarsest soils with very rigorous management. The quality of CBM produced water is not yet known in Alaska. Will CBM development reduce flow to streams, springs and wells?As a result of the large amount of water being pumped from the coal seam aquifers, there is concern of impact to springs and streams and to the level of water in drinking, and livestock wells. The answer to this question is very location specific. If the spring or stream is fed by the coal seam aquifer (the coal seam surfaces and discharges water into a stream or spring), CBM development in the local area may well decrease the flow to those water bodies. If the spring or stream is not fed by the coal seam aquifer, decreases in flow would be minimal. However, if CBM produced water is land applied or impounded in a holding pond (most often these ponds are not lined and discharge to the subsurface), streams downslope may have increased flow during development due to subsurface flow to the stream. If the drinking water or livestock well gets water directly from the coal seam, then CBM development in the local area may decrease the water level in that well. The duration of impacts to spring flow and water available from wells will depend on the total area developed and timing. I've heard people talk about aquifer drawdown from CBM development. What does this mean?Ground water flows through the coal seams due to water pressure, or hydrostatic head. When the pump in a well is turned on, the amount of water that can be produced is controlled in part by the static water level, which is the original hydrostatic head in the well. As the pump withdraws water from the aquifer and discharges it at the surface (whether it is to a stock tank, house, or CBM discharge point) the water pressure (head) in the aquifer is reduced. The greatest reduction in water pressure is near the well, with progressively less change at increasing distances from the well. If we could see this reduction in water pressure it would be shaped like a funnel or cone with the spout in the well. This area of reduced water pressure is called the cone-of-depression. When the pump is turned off, water flowing through the coal aquifer replaces the discharged water, and the water pressure returns static conditions. A ground water flow system, where the head shows the drawdown associated with CBM production is shown in the following figure.
Within the cone-of-depression, there is less water pressure in the aquifer, and therefore less water can be produced from a well (or spring). The percentage change is greatest near the central or deepest part of the cone-of-depression. The amount of change in water pressure and the distance from the producing well to the limit of change depends on many factors, including the static water level, pumping rate, aquifer characteristics, and how long water is produced. Also, the time needed for water pressure to return to static conditions is dependent on the same parameters. In cases where a field of producing wells, as is the case with CBM, the size of the cone-of-depression and the recovery time are both increased significantly. Some individuals say that the depleted aquifers in the Powder River Basin (Montana) will be recharged within a matter of years, while others think the time might be more on the scale of a thousand years or more. Who's right? Aquifer recharge is a process whereby precipitation or some other water source infiltrates directly into the formation holding water or water percolates through the soil to an aquifer. In the case of CBM, the coal seam is saturated and the recharge water must enter a clinker or scoria rock outcrop which may be miles away from where the development occurs. According to the Montana Bureau of Mines and Geology, monitoring and groundwater modeling indicates somewhere between a few years and 20 years for recharge to occur. The question of recharge time is a challenging one. In coal mining areas, recharge occurs within a few years (5 or maybe less). But the area of impact for open pit or strip coal mining is 1 to 5 square miles. With CBM extraction the area of impact may be as large as a township (36 square miles). Because the area of impact is relatively small with open coal mining and the total area contributing to recharge and storativity is large, recharge can occur relatively rapidly. However, in a large geographic area like that associated with CBM extraction, there is no longer enough water left in storativity after production to recharge the system to any significant degree. So recharge depends on the time it takes recharge at the coal seam outcrop to move to the CBM developed area (Wheaton, 2002). Are Coal Seams Aquifers?Yes. Water flows through fractures (or cleats) in the coal seam and if the cleat system is well developed and has enough water to pump and produce an economically viable and feasible water supply, the seam can be an aquifer.
References Cited:
Decker, M. K. (General Chairman), 2001, Potential Supply of Natural Gas in the United States: Report of the Potential Gas Committee December, 2000: Golden, CO, Potential Gas Agency, Colorado School of Mines Report, 346 p. Questions/Comments: waterquality@montana.edu, lois@inletkeeper.org |
