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Coal-bed methane product groundwater—disposal options and treatment Technologies
By Daniel P. Duffy
Millions of years ago when dinosaurs roamed the Earth, it never occurred to their walnut-sized brains that their dead bodies and the decaying mass of dead vegetation that they fed on would decompose and be compressed by layers of mud and rock overburden to become the fossil fuels that make modern industrial society possible. The giants that shook the earth during the Carboniferous through the Permian eras, 380 million to 245 million years ago, laid down their lives so you and I could light our homes and drive our cars to the grocery store. We are all familiar with the standard trio of fossil fuels they produced: solid coal, liquid petroleum, and natural gas.
More recently, a non-traditional form of fossil fuel is being extracted in ever-greater quantities, coal-bed methane. A potentially abundant energy resource, coal-bed methane already accounts for approximately 7% of total natural gas production in the United States. However, its extraction process results in the release of potentially large quantities of sodium as other contaminants of concern.
For many years a variety of traditional water treatment methods have been employed to deal with the wastewater discharged into groundwater as a result of coal-bed methane extraction. Unfortunately, common water treatment options, such as reverse osmosis and nanofiltration, do not always offer the most effective or economical option for coal-bed methane wastewater. The need for an economical treatment technology with a small footprint and the ability to treat water for surface-level use has inspired several companies to focus on developing alternative treatment methods for coal-bed methane wastewater discharge.
Creation of Coal-Bed Methane
Coal-bed methane was differently formed than the other fossil fuels. As dinosaurs and their vegetation went through the early stages of coalification—the mechanical and biological process that turns organic materials into coal—aerobic bacteria (bacteria that use oxygen for respiration) started the work of forming coal-bed methane. Their metabolism depleted what little oxygen was left in the organic mass and surrounding soil and produced carbon dioxide.
Once the oxygen was completely depleted, the aerobic bacteria were superseded by anaerobic bacteria (bacteria that do not use oxygen and, in fact, are poisoned by oxygen). Anaerobic bacteria reduce the carbon dioxide previously formed by their aerobic cousins and produced methane through respiration. In saturated, freshwater environments having little in the way of free oxygen gas, the initial aerobic stage is skipped completely and anaerobic bacteria go straight to work producing methane. The combination of methane production and increased overburden increases the applied pressure on the proto-coal bed over time. When the coal strata’s underground temperature reaches about 120°F to 125°F, most of the biologically generated methane has been created. By then, the coal has become sub-bituminous.
Mechanical and thermal processes involving the increase of heat with depth of coal can also generate coal-bed methane. Sufficient applied heat energy can cause the disassociation of organic molecules and their recombination as methane and other byproducts. However, this process tends to produce secondary amounts of coal-bed methane compared to the microbial process.
Inflowing groundwater accumulates above the coal seam and percolates back down into the coal. This results in the coal seam becoming saturated. The increased groundwater pressure entraps the methane within the coal bed, where it waits for millions of years to be extracted.
Extracting Coal-Bed Methane
Though it is marketed and utilized in the same manner as natural gas, coal-bed methane requires a different kind of extraction process than is used to extract natural gas. Coal-bed methane is trapped within the groundwater that flows through the coal seam due to the high-overburden pressures. So the key to extracting coal-bed methane is to reduce this pressure. This is done by pumping water out of the coal seam. In this situation, the top of the groundwater surface is at a much higher elevation than the top of the coal seam or a low-permeability stratum of overlying soil confines the coal seam. Coal-bed methane is not soluble in groundwater to any great degree, so it readily separates from the groundwater once the confining pressure is reduced.
The object of the pumping isn’t to completely remove groundwater from the coal seam. Instead of complete dewatering, enough groundwater is removed to reduce the confining pressure (the applied water head). Usually the pumping lowers the physical elevation of the groundwater to the top of the coal seam. The flow created in the groundwater toward the extraction well head also directs entrapped methane to the well point. The well itself is typically a casing with the interior pipeline directly attached to a pump for the extraction of groundwater. The outer casing carries away the methane that accumulates at the well point. There is no need for any separation or filtration mechanism since coal-bed methane readily separates from the groundwater (while contained within the groundwater, it does not dissolve into the groundwater). Usually a blower mechanism is attached to the pipeline connected to the gas casing of the extraction well. The blower applies negative pressure to the gas and expedites extraction.
The long-term extraction of coal-bed methane from a significant coal seam is a three-stage process. The initial stage is the dewatering stage. During this first stage, groundwater production starts at its highest rate and then rapidly declines. During this time, methane production starts at its lowest point and approaches its peak. The second stage of production sees stabilized methane generation hit its peak while groundwater extraction reaches minimum levels. The last and longest stage, the decline stage, has little in the way of groundwater extraction as methane production slowly declines. Eventually all of the methane that can be economically extracted is removed and the well field is shut down and abandoned.
Extracted Coal-Bed Groundwater Problems
Each extraction well removes groundwater from a coal seam at rates that vary from 5 to 20 gallons per minute. The variance is due to either the nature of the coal seam itself (existing groundwater pressure, depth to the coal seam, amount of water, and porosity and permeability of the coal seam) or the stage of methane production. The total amount of water that can be generated by a methane extraction well field can be considerable. Typically, a well field consists of one extraction well every 50 to 100 acres (depending on the local geology) for each underlying coal seam. For example, if a 200-acre well field is underlain by four separate coal seams at depths and with a geological structure that require one well every 50 acres, the well field would consist of at least 16 extraction wells. Assuming that each well initially pumps out groundwater at a rate of 20 gallons per minute, the well field at the start of operations would produce over 300 gallons per minute. This is equivalent to over 460,000 gallons per day or almost 62,000 cubic feet per day. This is enough to cover an acre with almost 1.5 feet of water every day. After the first stage of the well field’s operation, the production of each well may fall to less than 1 gallon per minute. But during that first stage, large quantities of poor-quality groundwater have to be managed. The poor quality of the groundwater produced during coal-bed methane extraction can be due to either its saline content or its sodium content.
It is not just the quantity of groundwater extracted (and the resultant potential problems with aquifer depletion); it is also the potential poor quality of the extracted discharge water that is a concern. Groundwater extracted during coal bed methane production can have very high saline and/or high sodium content. Highly saline water has high concentrations of dissolved salts. These are not just the familiar table salts (sodium chloride) but also other salts based on calcium, magnesium, sulfates, bicarbonates, and boron. Nearly all of these present significant environmental degradation problems and render the discharged groundwater not potable. Measured by total dissolved solids (calculated in terms of parts per million, or milligrams per liter), unacceptably high levels of salinity occur when the water adversely affects crops growth and yield. Salts carried by saline water have trouble leaching through fine-grained soils (silts and clays) instead of coarse-grained soils (sands and gravels) and can build up as encrusted deposits on the ground surface. This can result in extremely high local salt contents in the soil and greatly reduced overall soil permeabilities. Some crops and plants (such as barley, sunflower, and wild rye) are relatively tolerant to slat concentrations while others (potatoes, peas, and clovers) are very sensitive and easily impacted.
High-sodium (or “sodic”) groundwater has elevated levels of sodium, calcium, and magnesium. Unlike saline levels, sodic levels are not measured in terms of total dissolved solids. Instead, a special measurement has been developed by the US Department of Agriculture called the Sodium Absorption Ratio (SAR) based on the relationship to the amounts of sodium to calcium and magnesium in the groundwater. An SAR of 12 or higher is considered to be sodic. Sodium also poses a threat to plants and soil depending on its concentration and ratio to the groundwater’s saline content. This is most noticeable in soils having 30% or more clay.
Coal-Bed Groundwater Disposal Options
Once the methane has been extracted, stored, and delivered to market, the question remains as to what to do with the extracted groundwater. There are several methods based on the final disposition of the extracted groundwater (return to groundwater, discharge to surface water, use to irrigate crops, reduction through evaporation, or direct domestic/livestock use).
Returning the extracted groundwater back to groundwater-bearing strata involves either injection (directly back into the coal seam that it was extracted out of or into a nearby aquifer or suitable geological formation) or percolation from the ground surface back through the soil until it finds its own way back into underlying groundwater. This has the benefit of achieving aquifer recharge over time. However, considerable time may be required for percolation to reach groundwater layers depending on the local hydrogeology. Point discharges back into the groundwater can result in significant mounding of the groundwater level adjacent to the well point(s), which can also result in a significant delay before the mound height lowers and natural groundwater levels for the region are achieved. Furthermore, the reintroduction of extracted groundwater to another water-bearing strata may still result in local aquifer depletion as the operator “robs Peter to pay Paul.” But, the most serious potential impact of reintroduction of extracted coal-bed groundwater is contamination of the receiving groundwater and overlying soils. As described above, this water can be highly saline or sodic and either of these conditions could adversely impact the receiving groundwater, which may be otherwise potable or suitable for agricultural and industrial use.
Direct discharge of extracted groundwater to nearby bodies of surface water (ponds, lakes, streams, and rivers) is also a possibility. While this could have a positive impact on increased surface-water flows or as a source of water for feeding man-made or natural wetlands, these positive results are more than outweighed by the potential negative effects. Large amounts of discharge can alter existing surface-water flow patterns and potentially lead to spring and seep dewatering. And, as with reintroduction back to groundwater, there remains the significant harm from contamination of the receiving waters. This is an especially serious problem when freshwater lakes and streams receive large quantities of highly saline discharges that negatively impact local fish, vegetation, and wildlife.
Extracted coal-bed groundwater can be utilized for crop irrigation, provided extreme care is taken. As another source of irrigation water, the extracted groundwater can—at least in the short term—increase crop yields and the extent of native pasture. But as mentioned above, highly saline or sodic groundwater can adversely impact soil over time. Salts from the groundwater can accumulate in root zones, resulting in stunted plant growth as the plants extract less and less water from the soil. Encrusting of soil surfaces and the resultant clogging of soil pore spaces with sodium accumulations can severely reduce soil permeability and choke off soil aeration. Sodium also has the side effect of causing an increase in the degree of swelling by clays when wet. This causes a wider dispersion of heavy clay particles making the soil unsuitable for cultivation.
Miscellaneous methods of coal-bed groundwater disposal include man-made evaporation ponds (which can represent a significant capital investment and presents the problem of what to do with the brine residue) and direct use by domestic, livestock, and recreational or industrial consumers. Too often the degree of saline and sodic levels in the extracted groundwater renders it unsuitable for any direct use without significant (and potentially costly) treatment prior to use.
Coal-Bed Groundwater Treatment Options
There are about a dozen technologies that have been applied to the treatment of coal-bed methane produced groundwater:
- Reverse Osmosis. This procedure uses an applied pressure head to force water containing impurities through a porous medium or membrane. The impurities are retained on the upside of the membrane while pure water passes through the other side. While effective and a proven technology, this can be extremely expensive in large-scale applications. It also presents the problem and subsequent costs of how to properly dispose of the captured impurities.
- Nanofiltration. Like reverse osmosis, nanofiltration utilizes a membrane to separate out impurities from the produced groundwater. However, this process is more useful for groundwater with low total dissolved solids, not the high total dissolved solids that are characteristic of coal-bed methane groundwater.
- Ion Exchange. This utilizes the exchange of ions between two electrolytes immersed in the extracted groundwater. Ion exchangers include resins, zeolite, and montmorillonite clays and often used in household applications to produce soft water for domestic use. Large-scale applications, like those required for stage-one coal-bed methane extraction, become prohibitively expensive.
- Capacitive Desalination. This is a new technology for desalination that holds great promise but is still in the development stage. While capable of removing up to 7% of total dissolved solids, it is limited to water containing no more than 2,500 parts per million.
- Artificial Wetlands. These man-made wetlands utilize phyto-remediation (the use of plants that are capable of drawing up and retaining impurities from water). A proven technology with many applications from removal of heavy metals to the reduction of the acidity of mine-spoil discharges, land requirements for theses structures can be considerable (often hundreds of acres). Furthermore, the plant material that retains the impurities has to be regularly “harvested” and its contaminated vegetation mass safely disposed of.
- Electrodialysis Reversal. This technique is similar to ion exchange but is capable of exchanging both negatively and positively charged ions alternately. During this process, groundwater is forced through two membranes that can have their polarity reversed. This can be used to remove accumulated scales and allow for easy self-cleaning. But this process must be used with chemical additives and is difficult to scale up for large flow applications.
- Distillation. This is the traditional way of removing contaminants from groundwater and can achieve removal rates of up to 99.5%. However, a great deal of heat energy is required to boil away the pure water, leaving behind the contaminant-filled brine, and its energy cost can be too high for large-scale applications.
- Rapid-Spray Evaporation. This is a form of distillation that uses a rapid-spray system to separate pure water from its contaminants. This process ejects the contaminated groundwater through a nozzle into a stream of heated air. The heat from the air forms a mist of droplets that vaporize almost instantly, causing the separation of the good water from the bad impurities. Currently being developed, pilot projects utilizing this technology show great promise in reducing the costs of desalination.
What is needed is a proven technology that can be applied at a relatively small operational footprint at low costs per unit treated that can convert groundwater produced during coal-bed methane extraction into water suitable for surface-water discharge, livestock, irrigation, or even domestic use.
Eco-Tec and PureTech have formed a partnership with over 18 years of experience in the field of industrial mining to address the issue of coal-bed methane–produced water. Eco-Tec is a global manufacturer (providing over 1,500 systems to markets in 52 countries) of water purification and chemical recovery systems for industrial operations. PureTech was formed specifically to provide solutions to the coal bed methane industry with full, turnkey operations; maintenance services; hands-on engineering; and aquatic biology expertise. Eco-Tech’s manufacturing expertise is combined with PureTech’s technical expertise in the installation and commissioning of coal-bed methane water treatment systems.
Eco-Tech’s proven integrated technologies are based on its patented advanced ion exchange process, Recoflo, that offers significant cost reduction and superior process efficiency. Its new RecoPur coal-bed methane water treatment system is designed to recover high-sodium water and purify it to acceptable levels while converting the extracted waste and brine into useable and marketable byproducts. That’s the primary selling point of this new technology: No waste is generated.
According to Water Online, a water industry Web site and e-newsletter, the system is specifically configured to economically purify and recycle produced water that exceeds agricultural and environmental standards. RecoPur provides a continuous, online process for handling large volumes of water from multiple wells producing in excess of 50,000 barrels per day. At 42 gallons per barrel, this is equivalent to 2,100,000 gallons per day or about 280,740 cubic feet per day. This is equivalent to almost 6.5 acre-feet of water each day and can easily handle the largest water production systems. While handling such large quantities, it produces superior quality of treated water, achieving SAR levels of less than 3. This exceeds the purification standards for agricultural and other uses.
The system is a three-stage process, utilizing a micro media filter, a Recoflo cation exchanger, and a decarbonator. The Spectrum Micro Media Filter prevents fouling of ion exchange resin and ensures the performance of the cation exchanger. The filter can handle water with a suspended solids concentration in excess of 100milligrams per liter and provides low-filtrate turbidity. These results are achieved with two layers of specialized media. An upper layer of coarse material provides depth filtration while a lower, extra-fine layer removes very small particles. Water Online says that the Recoflo cation exchanger is used to remove all cations present in the water and can be regenerated with either sulfuric or hydrochloric acid depending on the market potential of the residual brine. It is this system that is key to the waste recovery aspects of the system. Recoflo is a short-bed ion exchange process. Standard applications using this process involve the recovery of nickel salts from rinse water, the removal of contaminants from chromic acid plating solutions, and the removal of metals from anodizing and etching baths. The decarbonator contacts the water with air to remove carbonates such as carbon dioxide. By converting the carbon dioxide, it removes the alkalinity from the groundwater solution.
By a unique combination of proven technologies, the RecoPur system offers large flow-through capacity with low unit operating costs in an easily transported, skid-mounted unit that can be delivered where needed. Often, it’s not the new breakthroughs that matter so much as insightful utilization of existing tools. Eco-Tech isn’t the only company that has discovered this basic principle.
Founded in 1987, Drake Engineering Inc. (DEI) is a woman-owned, Montana-based, private research and development firm. Until recently, it has been best known for providing on-demand capability in the disciplines of chemical, hydrogeological, nuclear, and civil engineering.In collaboration with Montana State University and the Department of Energy, DEI has developed a proprietary Coal-Bed Methane Water Treatment System. Tests of the new system are being performed at the Marathon Oil Corp.’s Powder River Basin site in Wyoming and should be completed this year. Their system allows “resource extraction in an environmentally responsible manner,” says co-owner Vivian Drake. The device measures 8 feet by 12 feet by 11 feet high, is skid-mounted, weighs 9,000 pounds and can be taken onsite by a pickup truck. This compact device can process 250 gallons per minute, about 8,500 barrels per day.
Like others looking for a cost-effective solution to coal-bed methane water treatment problems, RG Global emphasizes overall reduction of the wastestream. The company’s Catalyx water system is designed to remove sodium and other contaminants from the coal-bed methane water stream, making more than 99% of the water fit for use in irrigation. The remaining 1% largely consists of decahydrated sodium sulphate, which can be harvested and sold as the commodity Glauber’s salt. RG Global’s system allows for highly scaleable plant design that allows rapid operational changes in the flow rate of the water treatment capacity as needed. Catalyx-processed wastewater exceeds Montana’s new requirements for discharge of coal-bed methane wastewater. The system uses Catalyx Fluid Solutions Inc. patented ion exchange technology to remove inorganic contaminants such as sodium, barium, and iron from the coal-bed methane process water. Construction of the pilot facility began this year.
DANIEL P. DUFFY, P.E., is an environmental engineer employed by URS Corporation in Akron, OH.
OW - September/October 2007 |
