The primary energy source of natural gas is a substance called methane (CH4). Coal bed methane (CBM) is simply methane found in coal seams. It is produced by non-traditional means, and therefore, while it is sold and used the same as traditional natural gas, its production is very different. CBM 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, with methane is held in the coal by water pressure. Currently, natural gas from coal beds accounts for approximately 7% of total natural gas production in the United States.
How do people estimate the amount of methane gas which will come from a region underlain by coal? There are two popular methods of estimating recoverable methane gas from a coal seam.
a. One method requires estimating methane reserves by boring to the top of the coal seam, then extracting a core from the coal. The amount of methane recovered from the coal core is used to estimate gas content per unit volume of coal. If a number of cores are drilled and methane gas release is observed, one can estimate 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 measured;
2) it is expensive
3) not every region of potential CBM development has been drilled and explored
b. Another method of estimation is through a series of calculations based on information already known 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:
• A coal seam has favorable reserves if it produces 50-70 ft3 per ton of coal.
• CBM extraction is economical at 50 ft3 per ton of coal when a coal seam is 20 feet thick or more.
• Coal bed methane exists only in areas where the dominant chemistry of the water in the coal seam is sodium bicarbonate and where the coal seam is buried deeply enough to maintain sufficient water pressure to hold the gas in place.
The Environmental Impact Statement for CBM development in the Powder River Basin estimated the amount of coal in the region based on the total 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 outcrop.
How do gas companies extract methane from a coal seam?
Since CBM travels with ground water in coal seams, extraction of CBM involves pumping available water from the seam in order to reduce the water pressure that holds gas in the seam. CBM has very low solubility in water and readily separates as pressure decreases, allowing it to be piped out of the well separately from the water. Water moving from the coal seam to the well bore encourages gas migration toward the well.
CBM producers try not to dewater the coal seam, but rather seek 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.
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. Coal seams are the most regionally continuous geologic unit in the Powder River Basin and have aquifer characteristics equal to or better than sandstones, so are frequently targeted for water-well completions.
I've heard people talk about aquifer drawdown from CBM development. What does this mean?
Ground water flows through coal seams due to water pressure, or hydrostatic head. When the pump in a well is turned on, the amount of water than 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 to static conditions. An idealized ground-water flow system, and one
where the head shows the drawdown associated with CBM production are shown in the following figures.
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 with a field of producing wells, as is the case with CBM, the size of the cone-of-depression and recovery time are both increased significantly.
Some individuals say that the depleted aquifers in the Powder River Basin 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 is right?
Aquifer recharge is the process whereby precipitation or surface water infiltrates below land surface and begins to flow in an aquifer system. Ground water flowing through coal seams in the Powder River Basin has infiltrated along clinker or scoria ridges, in stream valleys, and in some cases in sandy soils during years of heavy precipitation. In the case of CBM product water, recharge occurs many miles away from development sites.
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 (typically 3 to 4). However, open pit or strip coal mines normally cover an area of only a few square miles, and because the area of impact is relatively small, recharge can occur rapidly. With CBM extraction, the area of impact may be as large as many adjacent townships (1 township=36 mi2). In such large geographic areas recharge depends on the time it takes recharge at the coal seam outcrop to move to the CBM developed area (Wheaton, 2002).
I heard that almost all CBM product water discharged to the land surface eventually reaches the aquifer from which it was pumped. Is this true?
MSU scientists contend that most likely only a small percentage of CBM water returns to the aquifers from which it was pumped. Rather, the water recharges shallow alluvium and coarse soil material aquifers near the land surface, less than 200 feet deep, or is lost to evaporation. If CBM product water is directly discharged into a stream channel (this is no longer allowed, but there are sites that were "grandfathered") it can flow downstream, evaporate, or percolate to groundwater through the stream channel. Land applied or stored CBM product water evaporates or percolates to shallow groundwater.
Once water reaches a shallow aquifer, where it goes is very site specific. The aquifer water pressure (head) may increase, and/or the water may flow laterally to a spring or become baseflow to a nearby stream. There are reports in the Powder River Basin that some stream channels are carrying more flow than before CBM development, and there are reports that some streams have no increase in flow. With our current level of knowledge, it is very difficult to predict what will happen to the water once it reaches the shallow aquifer system.
In Gillette, WY, the Pennaco company is reinjecting water back into the depleted aquifer which supplies water for the city of Gillette. Pennaco, and other companies in the Powder River Basin, are investigating the feasibility of injecting CBM product water at several sites in the area.
Will CBM development reduce flow to streams, springs and wells?
As a result of the large amount of water being pumped from 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 a spring or stream is fed by a coal seam aquifer (the coal seam surfaces and discharges water into a stream or spring), CBM development in the local area may decrease flow to those water bodies.
If a spring or stream is not fed by a coal seam aquifer, decreases in flow would be minimal. However, if CBM product water is land applied or impounded in a holding pond (most often these ponds are not lined and discharge to the subsurface), streams down slope may have increased flow during development due to subsurface flow.
If a drinking water or livestock well gets water directly from a coal seam, then CBM development in the local area may decrease the water level in that well. Duration of impacts to spring flow and water available from wells will depend on the total area developed and timing.
Source:
Frequently Asked Questions Coal Bed Methane (CBM)
Kristin Keith and Jim Bauder, Montana State University-Bozeman
John Wheaton, Montana Bureau of Mines and Geology (2003)
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