COAL BED GAS GENERATION

COAL BED GAS GENERATION

NARMEEN HAROON

ROLL NO: 12

4TH SEMESTER

CENTRE FOR COAL TECHNOLOGY

UNIVERSITY OF THE PUNJAB

LAHORE.

What is Coal Bed Gas ?

Coal bed gas also known as Coal bed methane (

CBM ) 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.

2COAL BED GAS GENERATION

The methane is in a near-

liquid state lining 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.


Composition Of CBM

The composition of methane gas from coal

mines is variable but it is essentially a high

concentrated methane gas as shown below:

CH4 = 93 – 99 %

C2 H6 = 0 – 3 %

CO2 = 0 – 4 %

N2 = 2 – 6 %

Hydrogen and Rare gases are present in traces.

How CBM is formed

CBM is generated either through

chemical reactions or bacterial action.

Thermogenic Production.

Biogenic Method.

High Rank Coal.

Low Rank Coal.


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).



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.


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 CBM 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.


CBM is removed by removing water pressure which holds CBM in place. Methane that was held in place by water pressure tends to follow the water as it is pumped to the surface, where it is captured and transported through pipelines. Fracturing fluids are often first injected into the coal bed to break up the coal, making it easier for the water and gas to flow to the surface.

How is coal bed methane removed?


Gas

Water

(production

fluid)

Coal Bed


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.


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).


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.


CBM wells are required to be spaced at least 1000 feet apart.

The 1000 feet distance is based on what is called the “cone of

depression.”

Pumping from wells lowers the water table.

This area is known as the cone of depression.

Groundwater flow is diverted towards the well

as it flows into the cone of depression.16COAL BED GAS GENERATION

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 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 onstandards used for irrigation suitability.

Irrigation with water of CBM product water quality onrange or crop lands should be done with great care andmanaged closely.

With time, salts from the product water can accumulatein the root zone to concentrations which will affect plantgrowth.

Saline conditions stunt plant growth because plantsmust work harder to extract water from the soil.

the production fluid is subjected to pH adjustments andaeration if necessary and settling. Once the water hasbeen treated, it can then be discharged.


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 riparian and wetland

High salinity and sodium levels in product water may alter plant communities by causing replacement ofsalt 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

Australia

• Bowen Basin, (Fairview, Scotia, Spring Gully), Queensland, Australia

• Surat Basin, Berwyndale, Windibri, Kogan, Daandine, Tipton West, Queensland, Australia

Canada

• Telkwa coalfield, British Columbia

• Western Canadian Sedimentary Basin, Alberta

South Africa

• Molteno Coal Field, Eastern Cape

United Kingdom

• Cheshire, Lancashire, Staffordshire

United States

• Appalachian Basin, Alabama, Pennsylvania, Ohio, Wyoming , Colorado and New Mexico


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, British Columbia 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 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.


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

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.28COAL BED GAS GENERATION

Recommend: Most Sustainable

Practice Reinjection into aquifers depleted by

CBM production.

Injection or percolation into depleted aquifers with water treatment as required protecting and/or enhancing water quality.

Replace other uses where quality allows.

Surface discharges with water treatment as required resulting in improved stream flows.


References:

www.wikipedia.org

http://www.ml.com/media/43347.pdf

www.eurenergyresources.com

http://www.eia.doe.gov/


http://www.eia.doe.gov/

CBM- GAS OF THE PAST, PRESENT

AND THE FUTURE


COAL BED GAS GENERATION

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