What is coal bed natural gas?
The primary energy source of natural gas is a substance called methane (CH4). Coal bed natural gas (CBNG) 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. CBNG 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 natural gas is held in the coal by water pressure. Currently, natural gas from coal beds accounts for approximately 8% of total natural gas production in the United States.
Where is coal bed natural gas produced?
According to the Energy Information Agency, 13% of the land in the lower 48 United States has some coal under it, and some of this coal contains natural gas - either in the form we know as traditional natural gas or as CBNG. 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 CBNG. 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 CBNG 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 CBNG resources may lie below the Powder River basin of Montana and Wyoming (Decker, 2001).
How much natural gas is estimated will be extracted from the Powder River Basin?
Estimates of amounts of natural gas 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 CBNG 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 CBNG 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 CBNG in the Powder River Basin of Montana. The Montana statewide EIS for CBNG development in the Powder River Basin estimated 2.5 TCF of recoverable gas.
How do gas companies extract natural gas from a coal seam?
Since CBNG travels with ground water in coal seams, extraction of CBNG involves pumping available water from the seam in order to reduce the water pressure that holds gas in the seam. CBNG 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.
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.
What does aquifer drawdown from CBNG development 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 CBNG 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.
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 CBNG, the size of the cone-of-depression and recovery time are both increased significantly.
What is the recharge rate of a coal seam aquifer?
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 CBNG produced 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 CBNG 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 CBNG developed area (Wheaton, 2002).
Will CBNG 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), CBNG 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 non-existent or minimal.
If a drinking water or livestock well gets water directly from a coal seam, then CBNG 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.
What is the major concern with CBNG produced water?
There are several concerns about CBNG development and how to manage the water co-produced with natural gas.
The quantity of the CBNG produced water:
Extraction of CBNG involves pumping large volumes of water from saturated coal seams typically at rates of 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, it is anticipated that on average, three coal seams will be developed within a given area. Therefore, there may be several wells per 80 acres.
The quality of CBNG produced water and its effects on soil:
CBNG produced water typically has a moderately high salinity and more sodium than recommended for irrigation on clayey soils. Irrigation with water of CBNG produced water quality on range or crop lands should be done with great care and managed closely. With time, salts from the produced water can accumulate in the root zone to concentrations which will affect plant growth.
The sodium content of CBNG produced 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 CBNG produced water can degrade the physical and chemical properties of heavier, clay soils, making such soils unsuitable for plant growth.
What is saline water and why is it considered saline?
Saline water has a relatively high concentration of dissolved salts. Salt is not just "salt" as we know it - sodium chloride (NaCl) - but can be dissolved calcium (Ca2+), magnesium (Mg2+) sulfate (S042-), bicarbonate (HC03-) and Boron (B).
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.
What is sodic water and why is it considered sodic?
Sodic water is high in the sodium (Na+) concentration relative to concentrations of calcium (Ca2+) and magnesium (Mg2+). Sodicity of water is expressed as the Sodium Adsorption Ratio (SAR) which is:
The U.S. Department of Agriculture defines water with a SAR greater than 12 as sodic.
Are some soils more sensitive than others to saline and/or sodic water?
Yes, irrigation water that is suitable for one soil may not be for another. Use of saline and/or sodic water for irrigation can be risky business on soils predominated by silt or clay. Just 1 acre-foot of moderately saline irrigation water (EC = 3 dS/m - the upper end of suitability for irrigation water) will introduce 1.8 tons of salt to an acre of land. Soluble salts do not leach as readily through fine textured soils as through sandy soils. Therefore, when irrigating fine textured soils with moderately saline water, it is critical to add enough water to meet crop water requirements and to maintain a net downward movement of water through the soil.
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.
Saline Irrigation Water: EC = 3.2 dS/m, SAR = 5.0 slight risk
Sodic Irrigation Water: EC = 1.4 dS/m, SAR = 13 slight to moderate risk
Saline-Sodic Irrigation Water = EC 4.0 dS/m, SAR = 20 slight risk
What are the current management practices for disposal of CBNG produced water?
Currently, CBNG produced water in the Powder River Basin is primarily managed by the following methods:
- Untreated Discharge - Discharge to streams is allowed so long as a NPDES permit has been issued. Any discharge must comply with the applicable surface water standards.
- Treated Discharge - Currently several methods of water treatment are being used or proposed in the Powder River Basin. These include ion exchange, zeolite treatment, and reverse osmosis among others. There is presently one ion exchange CBNG treatment facility operating in Montana.
- Impounded - This method involves constructing a pond in which CBNG produced water is stored. Depending on the design of the impoundment, water may infiltrate to the subsurface and/or evaporate. In some cases, enhanced evaporation processes are used. There are several terms for these impoundments: "holding ponds", "zero discharge ponds" or "infiltration ponds".
- Land applied to crop or rangeland - through some form of irrigation equipment.
- Managed Irrigation – This method involves the application of agronomic principles to use CBNG produced water to produce forage while protecting the soil's physical and chemical properties.
- Beneficial Uses - CBNG produced water is used for stock water, dust control, drilling water and, in some cases, by coal mines.
Another option proposed for disposal of CBNG produced water in eastern Wyoming and Montana is to inject the CBNG produced water back into an aquifer(s). This practice occurs in the southwest U.S., where CBNG produced water is injected into formations below CBNG-bearing coal. This approach avoids surface discharge. Many opinions exist, and the feasibility - economic, physical, and environmental - of either injecting CBNG produced water to the coal seam from which it was pumped or injecting it into an aquifer above or below the CBNG-bearing coal seam is being investigated.