COAL BED METHANE RECOVERY & ITS POTENTIAL THREATS

Anubhav B.Tech in Mining Engineering (3rd year)University of Petroleum & Energy Studies

WHAT IS COAL BED METHANE ?• Coal bed methane is a natural gas by-product of coal

formation. During the coal formation,organic mattter is chemically broken down into simple organic compounds.

• Methane is a by-product of this breakdown.• Due to coal’s porous nature, methane gas produced

during coal formation is adsorbed into the coal bed and held in place by the weight of the surrounding groundwater.

• it is a gas that occurs in association with coal. During the coalification process, large amount of gas are produced, some of this gas escapes into other rocks or into the atmosphere but some still remain in coal.

• Permeability in coal is created by naturally occuring fractures referred as cleats

• Coal bed methane is similar to natural gas, differing only in the way that it is formed and stored in the Earth's crust

• Unlike much natural gas from conventional reservoirs, coal bed methane contains very little heavier hydrocarbons such as propane or butane, and no natural gas condensate. It often contains up to a few percent cabon dioxide.

METHANE STORAGE IN COAL

Methane in coal is:

• Adsorbed on the surfaces of the coal

• Stored as free gas in the cleats and open pores

• In the coal layer, there are many fractures (cleats), which formed during the process

• water and gas flow in the coal layer through the fracture(cleat)

• The part of the coal which is surrounded by the cleat is called the matrix (coal matrix), where most of the cbm attaches to the pores in them.

THE CHARACTERISTIC OF COAL BED METHANE

1.PorosityThe porosity of coal bed reservoirs is usually very small, ranging from 0.1 to 10%.

2. Fracture permeabilitythe fracture permeability acts as the major channel for the gas to flow

3.Thickness of formation and initial reservoir pressureThe thickness of the formation may not be directly proportional to the volume of gas produced in some areas

4. Other properties coal density, initial gas phase concentration, critical gas saturation

Cleats in Coal

Face Cleats

Butt Cleats Cleats are natural fractures in coal. Methane and other coal seam gases will flow out of pores of coal if there is a pressure gradient acting as a driving force and the fractures are sufficiently open or permeable.

HOW IS COAL BED METHANE EXTRACTED?

• In order to commercially produce coal bed methane, it is necessary to decrease the water pressure within the coal bed.

• When the pressure is decreased, the methane gas naturally desorbs from the coal and migrates through the coal bed.

• To obtain CBM, wells production made by drilling down from the surface to the coal layer

In the Coal Bed itself, coal has high pressure then the effect of the pressure drop will occur when groundwater is pumped around the coal layer (dewatering) to the surface. This will cause the methane gas released from the coal layer and then it will flow to the surface through the wells production

The Three stages of CBM Production

Time

MCF

D or

BPD

Water Gas

Stage 1,De-watering

Stage 2,Mid Life

Stage 3,Decline production

ECBM (ENHANCED COAL BED METHANE RECOVERY)

• ECBM (Enhanced Coal Bed Methane Recovery) is a technique to improve CBM which is produced.

• In this technique, gas injection is commonly used is N, and CO2. Here, the results obtained are very different depending on which gas injection is used.

AFFINITY OF H2S WAS MORE THAN 10 TIMES OF METHANE !

Reservoir Pressure Psi

Gas Co

nten

t

Methane

CO2

H2S

CO2

H2S

Methane

Using CO2 for pressure maintenance can also reduce CO2 emissions (sequestration).

CO2 Injection

Methane Production

IMPACTS OF METHANE EXTRACTION

• Ground water depletion• Salt water intrusion• Surface water discharge• Chemical contamination• Large scale environmental effects

IMPACTS TO ENVIRONMENT• Ground Water Depletion.• One of the environmental effects of coal bed methane extraction is the immense quantity of water

pumped out of the coal bed aquifers.• On average, approximately 12-15 gallons of water per minute are pumped from each well.

During the initial phase of production, water is pumped at a very high rate• Due to large quantity of groundwater extraction the water table goes down results in

groundwater depletion.• CBM wells are connected by a network of roads, pipelines, and

compressor stations. Over time, wells may be spaced more closely in order to extract the remaining methane. Additionally, the produced water may contain undesirable concentrations of dissolved substances. Water withdrawal may depress aquifers over a large area and affect groundwater flows.

• In several reported cases, local water wells have gone dry after coal bed methane operations have begun.

SALT WATER INTRUSION:

IMPACTS DUE TO SURFACE WATER DISCHARGE

• Surface discharge is the most common disposal option for the extracted coal bed water, the compositional characteristics of coal bed water can have a tremendous impact on the surrounding ecology.

• The quality of coal bed water varies considerably from well to well and basin to basin, but, on average, the deeper the coal bed, the more saline the water becomes.

CONTINUED…

• Other compositional elements typically seen in extracted coal bed water include:

• Major Cations (positively charged ions such as sodium, potassium, magnesium, & calcium)

• Major Anions (negatively charged ions such as chlorine, sulphate, & hydrogen carbonate)

• Trace Elements & Metals (iron, manganese, barium, chromium, arsenic, selenium, & mercury)

• Organics (hydrocarbons and additives).

CONTINUED…

• The saline and sodic quality of coal bed water can have catastrophic impacts on local agriculture when discharged into local waterways.

• The moderate to high levels of salt in coal bed water can destroy soils and decrease crop production.

• The salts gather in the root base of plants, making it harder for the plants to extract water from the soil and inhibiting growth.

CHEMICAL CONTAMINATION

• One of the process used for methane extraction is Hydraulic Fracturing.• Hydraulic fracturing pumps a mixture of heavy chemicals, water, sand

and/or other materials down an extraction well under extremely high pressure in order to achieve the desired fracturing.

CONTINUED…

• Hydraulic fracturing raises serious concerns because of the chemicals being used and their impact on the local ecology.

• If the coal bed aquifer is tributary to surface water or other groundwater aquifers, chemical contamination can spread into domestic, agricultural, and industrial water supplies.

• Some of the chemicals used for this process is Lead, Uranium & Mercury which are all carcinogens.

LARGE-SCALE ENVIRONMENTAL EFFECTS

• Methane exists in the environment as a greenhouse gas that has been linked to global warming.

• After carbon dioxide, methane is the second most important greenhouse gas. Methane's warming effect is 21 times greater than that of carbon dioxide; however, it stays in the atmosphere for a significantly shorter period of time.