FAQs: Coal Bed Methane
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.
According to the CBM Association of Alabama, 13% of the land in the lower 48 United
States has some coal under it, and some of this coal contains methane - either in
the form we know as traditional natural gas or as CBM. According to the United States
Geological Survey, the Rocky Mountain Region has extensive coal deposits bearing an
estimated 30-58 trillion cubic feet (TCF) of recoverable CBM. While impressive, this
represents only one third of the total 184 TCF of natural gas in the Rocky Mountain
region (Decker, 2001).
Within the Rocky Mountain Region, untapped sources of CBM exist in the Powder River Basin of Wyoming and Montana, the Greater Green River Basin of Wyoming, Colorado, and Utah, the Uinta-Piceance Basin of Colorado and Utah, and the Raton and San Juan Basins of Colorado and New Mexico. An estimated 24 TCF of recoverable CBM resources may lie below the Powder River basin of Montana and Wyoming (Decker, 2001).
Estimates of amounts of methane gas in the Powder River Basin vary and are often re-calculated.
There are several methods to estimate the amount of recoverable gas from a coal seam,
all having varying degrees of accuracy.
According to the U. S. Geological Survey (2001), the amount of recoverable CBM in the Powder River Basin ranges from 8.24 - 22.42 TCF. The Wyoming Oil and Gas Conservation Commission (2002) estimates 31.8 TCF of recoverable CBM in the Powder River Basin of Wyoming alone. The Montana Bureau of Mines and Geology and the U.S. Department of Energy have separately estimated 0.8 - 1.0 TCF of recoverable CBM in the Powder River Basin of Montana. The Environmental Impact Statement for coal bed methane development in the Powder River Basin of Montana reports 2.5 TCF of recoverable gas.
There are two popular methods of estimating recoverable methane gas from a coal seam.
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 and 3) not
every region of potential CBM development has been drilled and explored.
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, regardless of seam thickness, depth or proximity to outcrop.
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.
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.
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.
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).
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.
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.
There are several concerns about CBM development and how to manage the water co-produced with methane.
The quantity of the CBM product water:
Extraction of CBM involves pumping large 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 this volume of often marginal-quality CBM product water is a source of much debate. Each well produces 5 to 20 gallons of water per minute. At 12 gallons per minute, one well produces a total of 17,280 gallons of water per day. It is common to have to have one well every 80 acres, and in the Powder River Basin, there are up to three methane-bearing coal seams. Therefore, there may be up to three wells per 80 acres.
The quality of CBM product water and its effects on soil:
CBM product water has a moderately high salinity hazard and often a very high sodium hazard based on standards used for irrigation suitability. Irrigation with water of CBM product water quality on range or crop lands should be done with great care and managed closely. With time, salts from the product water can 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 product water poses additional threats to certain soil resources. Sodic irrigation water causes soil crusting and impairs soil hydraulic conductivity, adversely affecting water availability and aeration and subsequent crop growth and yield. Upon wetting of soils containing swelling clay, sodium causes the degree of swelling in the clay to increase, leading to dispersion and migration of clay particles. Current research at Montana State University shows that water with sodium levels equal to typical Montana CBM product water can degrade the physical and chemical properties of heavier, clay soils, making such soils completely unsuitable for plant growth.
The risk of sodium degradation has been observed in other soil textures. Jim Oster
(personal com.) observed crusting, poor soil tilth, hardsetting and aggregate failure
on a sandy loam soil irrigated with water with EC ~ 1 and SAR ~ 7. Minhaus (1994) saw irreversible and severe reduction in infiltration on sandy loam
soil with long term irrigation under high SAR water followed by monsoon rain.
There are many factors in addition to soil textures that affect infiltration rates. Mineralogy, lime, sesquiozides, organic matter content, cultivation, irrigation method, wetting rate, antecedent water content and time since cultivation all play a roll in infiltration. The only way to be certain of the impacts of saline/sodic irrigation water on the soil is to periodically sample and test the irrigation water and the soil.
The quality of CBM product water and its effect on plants:
Disposal of the quantities of CBM product water into stream channels and on the landscape poses a risk to the health and condition of existing riparian and wetland areas. High salinity and sodium levels in product water may alter riparian and wetland plant communities by causing replacement of salt intolerant species with more salt tolerant species. It is well recognized that encroachment of such noxious species as salt cedar, Russian olive, and leafy spurge is enhanced by saline conditions.
Salinity of water is referred to in terms of Total Dissolved Solids (TDS) and can be estimated by measuring Electrical Conductivity (EC), expressed as decisiemen per meter (dS/m), or less often in millimhos per centimeter (mmhos/cm) (the two measurements are numerically equivalent). EC is also reported in microsiemens or micromhos per centimeter, equal to 1,000 times dS/m. TDS is approximately related to EC by the following equations:
- TDS (parts per million, ppm or milligrams per liter, mg/l) = 640 x EC (dS/m)
- TDS (milliequivalents per liter) = 10 x EC (dS/m)
Water is considered saline when it becomes a risk for crop growth and yield. The U.S. Department of Agriculture defines water with an EC greater than 3.0 dS/m as saline.
SAR = Na √ [(Ca + Mg) / 2] (These values are in meq/L)
The U.S. Department of Agriculture defines water with a SAR greater than 12 as sodic.
In addition to being a salinity component of irrigation water, sodium poses a more troublesome problem in soils with more than 30% clay. On such soils, sodium degrades soil physical properties, leading to poor drainage and crusting. Irrigation of sandy soils with sodic water on sandy soils does not cause such problems, as the sodium is more readily leached from the soil profile.
Sodium risk to soil infiltration cannot be determined solely from the USDA definition of sodic water (SAR = 12). Therefore, the sodium hazard of irrigation water on soil infiltration must be determined from the SAR/EC interaction. Ayers and Wescot (1985) outline guidelines for evaluating sodium risk to soil infiltration. The risk is soil texture independent. The three examples below illustrate the need to evaluate the risk to soil infiltration based on the EC/SAR interaction. It is important to understand that rainfall or irrigation with non-saline water on soils previously irrigated with saline sodic water can increase the sodium hazard by lowering the EC much faster than the SAR.
Irrigation of crops with water of CBM quality can be risky if not managed closely. With time, salts accumulate in the root zone to concentrations which will affect crop growth. Saline conditions stunt plant growth because the plants must work harder to extract water from the soil. Table 1 illustrates salt tolerance levels for some common Montana crops.
Elevated sodium and chloride concentrations can harm some woody plants as they are
taken up by the root cells or directly applied to plant leaves. Either way, ions can
accumulate in the leaves, causing leaf burn along the outer leaf edges.
Sodium can indirectly affect crop growth by causing calcium, potassium, and magnesium deficiencies or by adversely affecting soil physical properties. If irrigation water is sodic, physical properties of heavier soils (predominantly silt or clay) may be altered, affecting the soil's ability to drain. Poorly drained soils can compromise crop growth and yield.
To avoid build-up of salt in the soil, annually leach the soil with enough non-saline water so the salts are moved below the root zone. Adequate drainage is absolutely necessary for this procedure to be successful. Research in the western United States has shown that substantial volumes are needed to leach salt from the soil.
Currently, CBM product water in the Powder River Basin is managed by the following methods:
- Discharged into a stream channel - Although direct stream discharge is no longer permitted on new wells, existing operations were "grandfathered" and are still discharging directly into streams. Also, proposals are being advanced to allow regulated discharges during certain flow conditions.
- Impounded - This method involves constructing a pond in which CBM product water is stored or allowed to infiltrate to the subsurface. There are several terms for these impoundments: "holding ponds", "zero discharge ponds" or "infiltration ponds". Although they do not directly discharge water on the land surface, most impoundments are not lined and do discharge to the subsurface. Some percentage of seepage flow from impoundments is likely to reach stream channels via subsurface flow.
- Land applied to crop or rangeland - through some form of irrigation equipment
- Other uses - CBM product water is also used for dust control and, in some cases, is being used by coal mines.
Another option proposed for disposal of CBM product water in eastern Wyoming and Montana is to reinject the CBM product water back into an aquifer(s). This practice occurs in the southwest U.S., where CBM product water is injected into formations below CBM-bearing coal. This approach avoids surface discharge. Many opinions exist, and the feasibility - economic, physical, and environmental - of either reinjecting CBM product 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.
It is possible to alter the chemistry of sodic water by adding calcium and magnesium. This does not eliminate or reduce sodium, but changes the ratio of sodium to other salts, thus decreasing the sodium adsorption ratio (SAR). The net result is more saline water with the sodium salt still dissolved in the water. This approach is not likely to work with CBM product water because the added calcium will combine with carbonate from the CBM water and precipitate out as calcium carbonate (lime). To make this process work, CBM product water must be de-gassed of carbonate by addition of acid, or additional calcium must be made available in the soil by acidification from sulfur additions. Unfortunately, addition of more salts to water or soil may result in conditions too saline for plant growth.