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What is CBM ? Coal bed methane ( CBM ) also known as coalbed gas is a form of natural gas extracted from coal beds. The term refers to methane adsorbed into the solid matrix of the coal. Methane is stored within the coal by a process called adsorption. It is called 'sweet gas' because of its lack of hydrogen sulfide. Coal bed methane contains very little heavier hydrocarbons.
The methane is in a near-liquid statelining the insides of the pores within the coal (called the matrix). The open fractures in the coal (called the cleats) can also contain free gas or can be saturated with water. During the extraction, as the pores shrink, the overall matrix shrinks as well, which may eventually increase the space the gas can travel through (the cleats), increasing gas flow.
How CBM is formed? CBM is generated either through chemical reactions or bacterial action. Thermogenic Production. Biogenic Method. High Rank Coal. Low Rank Coal.
Answers to the frequently asked questions regarding CBM
Where does CBM exists? 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).
and Wyoming (Decker, 2001).
How the estimation is done? 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. 3) not every region of potential CBM development has been drilled and explored. :
Another method is through a series of calculations based on information 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.
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.
A Glimpse of CBM
Intrinsic Properties Affecting Gas Production Gas contained in coal bed methane is mainly methane and trace quantities of ethane, nitrogen, carbon dioxide and few other gases. Porosity. Adsorption Capacity. Fracture Permeability. Thickness of formation and initial reservoir pressure. Other Properties.
Methane will stay in a coalbed as long as the water table remains above the gas saturated coal.
Gas is released from the coalbed when cleat pressure is reduced by dewatering.
Some wells may never become economic if coals can’t be dewatered
Extraction A steel-encased hole is drilled into the coal seam (100 – 1500 meters below ground). Gas and produced water. Then the gas is sent to a compressor station and into natural gas pipelines. Coal bed methane wells often produce at lower gas rates than conventional reservoirs, typically peaking at near 300,000 cubic feet (8,500 m3) per day (about 0.100 m³/s).
A Typical CBM
Fate of CBM product water-(a) Quantity of 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. 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.
(b)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.
Effect on 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.
Left: An example of soils of eastern Montana that are high in swelling (montmorillonitic) clay. Right: Complete dispersion of the same soil following a season of exposure to high saline/sodic water.
The current management practices for disposal of CBM product water? 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.
Areas of Coal Bed Methane
Reserves A U.S. Geological Survey predicts more than 700 trillion cubic feet (20 Tm³) of methane within the US. At a natural gas price of US$6.05 per million Btu (US$5.73/GJ), that volume is worth US$4.37 trillion. . At least 100 trillion cubic feet (2.8 Tm³) of it is economically viable to produce. In Canada, BritishColumbia is estimated to have approximately 90 trillion cubic feet (2,500 km3) of coal bed gas
CBM as an Energy Resource Coal bed Methane Moves from Unconventional to Mainstream Energy Resource. Coal bed methane is now a significant part of our Nation's natural gas supply and less methane is released to the atmosphere. Currently considered a non-renewable resource There is evidence by the Alberta Research Council, Alberta Geological Survey and others showing coal bed methane is a renewable resource.
Coal Bed Methane in India India has huge Gondwana and Tertiary coal deposits distributed in several basins. The major part of Indian Gondwana coals (mostly up to 300 m depth) is of low rank. Tertiary coals of India is better in quality compared to Gondwana coals. The estimated coal bed methane resource of Gondwana coals appears to be between 1 and 1.5 Tcm and the Tertiary coals of about4.3 Bcm. GSI, Reliance Industries Limited have also undertaken investigations on the prospects of occurrence of coal bed methane in different Gondwana and Tertiary coalfields of India.
Coal Bed Methane in India Great Eastern Energy Corporation (GEECL) a part of YKM Holdings Group and the pioneers in bringing in technology for exploration and development of Coal Bed Methane makes CBM available for commercial sale purpose for the first time in India at Asansol. With the completion of the drilling of 23 vertical production wells by GEECL, Coal Bed Methane would be available in India for commercial sale purpose from 14th of July’07 priced at rupees 30 per Kg for CNG. GEECL is also setting up the first CBM station in India in the city of Asansol in West Bengal.
Environmental Impacts CBM wells are connected by a network of roads, pipelines, and compressor stations. Water withdrawal may depress aquifers over a large area and affect groundwater flows The release of CBM into the atmosphere adversely affects the global climate. Operators are required to obtain building permits for roads, pipelines and structures, obtain wastewater (produced water) discharge permits, and prepare Environmental Impact Statements.
Facts Coal is the world′s most abundant energy source Coal is a major source of hydrocarbons such as methane gas When plant material is converted into coal it generates large quantities of methane-rich gas Methane gas is then stored within the coal beds making coal a reservoir as well as a gas source It is estimated that coal and methane gas can be found in 13% of all the land in the lower 48 states of the US. Coal bed methane gas accounted for almost 8% of the U.S. natural gas demand.
Coal bed methane is currently a huge undeveloped energy resource Coal bed methane can be used as an clean energy source It is a safe, efficient and an environmentally more acceptable energy source Over the last two decades, the development of domestic natural gas supplies declined while consumption increased. There is now greater world market demand for cleaner fuels like Coal Bed Methane Gas and Natural Gas.
CBM- Gas of the past, present and the future Thank you