Coal Bed Methane

MOHAMMED KABIRUDDIN, MS-ES University of Petroleum & Energy Studies

What is Coal Bed Methane???  Coal Bed Methane (CBM) is naturally occurring methane (CH4)

with small amounts of other hydrocarbon and non hydrocarbon gases contained in coal seams as a result of chemical and physical processes.  CBM is an environment-friendly clean fuel with properties similar to natural gas. It is often produced at shallow depths and in most cases with large volumes of water of variable quality.

Coal Bed Methane  It is called 'sweet gas' because of its lack of hydrogen sulphide.

The presence of this gas is well known from its occurrence in underground coal mining, where it presents a serious safety risk.  Coal bed methane, often referred to as CBM, is distinct from a

typical sandstone or other conventional gas reservoir, as the methane is stored within the coal by a process called adsorption. The methane is in a near-liquid state, lining the inside of 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.

Reservoir mechanisms Coalbed Adsorption Phenomenon

Source: Reeves

Reservoir mechanisms

Source: Reeves

Factors for Exploitation  Porosity plays an important role in building up methane gas

reserves in the coal bed. Unlike the conventional reservoirs, in coal the methane is not compressed in the pore space (porosity) but physically attached to the coal at molecular level (micro-porosity).  Micro-porosity makes up about 70 percent of the total porosity in

coal bed and is equivalent to a conventional reservoir having 20 percent porosity, saturated with 100 percent gas. On account of this difference, coal has higher gas storage capacity than sands containing petroleum gas.

Reforming Process  Coal is a carbon-rich material that has been formed by the

chemical and thermal alteration of organic debris. During this process called coalification, a series of by-products are generated, including water and methane.

 With the progress of coal in rank from peat to anthracite,

about 140 m3 of methane is generated per ton of coal.

Reforming Process  Methane can be extracted from the coal seams by the

process of desorption according to which the initial reservoir pressure is reduced, by dewatering, to the critical desorption pressure. Thereafter, the coal seams release methane gas as the pressure is reduced.  The abandonment pressure is the lowest pressure at which

no more methane can be produced.  Before an exercise of drilling for the purpose of methane

extraction can be undertaken, an estimate of the reserves of coal bed methane gas is made.

Exploitation of CBM  The existence of gas in coal has been known for many decades. It is

only in the last decade and a half that this gas has emerged as a viable energy source with coal as both source and reservoir rocks.

 In USA, the CBM exploration was first initiated and an energy

resource has also been recognized. By 1995, USA has produced about 2.5 Bcfd (billion cubic feet per day) of CBM from 9000 wells, which is about 5 percent of the total gas consumption of USA.

 In CBM exploration, China is emerging as a major player and

Australia is on the threshold of commercial production.

CBM - Identification of Potential Coal bed Methane Area  The studies for delineation of blocks for prospecting / exploitation

of Coal bed Methane (CBM) have been carried out for

     

Jharia, Raniganj, East Bokaro, West Bokaro, North Karanpura and Sohagpur

 Coalfields by Central Mine Planning & Design Institute (CMPDI) at

the instance of Ministry of Coal and Directorate General of Hydrocarbons (Ministry of Petroleum & Natural Gas).

 Each of the above coalfields has been divided into two categories in

regards to prospecting / exploitation of CBM as given below :

 "YES AREA" : where CBM operations can be taken-up.  "MAY BE AREA": where coal mining may be taken up.  Hence, these blocks can be released for Coal Bed Methane only in

future subject to consent of concerned coal company / CIL.

 It would be seen from Table that a total of 1924.42 Sq. Km have been

delineated as "Yes Area" which is available for CBM exploration and exploitation in above mentioned coalfields. Based on these studies, Ministry of Petroleum & Natural Gas is likely to invite global bids for exploration / exploitation of CBM in India.

Steps in drilling a CBM gas well Geophysical inputs  Remote sensing imagery  Seismic survey 2. Drilling 1.

Remote sensing imagery  Remote sensing data to identify the major tectonic set-up

which is useful to explore an area of coal bed methane. The details of major fault trends will help in prioritizing the areas available for exploration.

Seismic survey  High resolution seismic survey is helpful to know the basin

configuration, its tectonic style, thickness of coal-bearing formation, lateral continuity and approximate depth of different coal seams.  The sampling interval, geophone, charge size, charge depth, group

interval and shot interval should be carefully chosen through experiments and the full spectrum of recorded frequencies during processing of data should be retained. The geophysical inputs may, however, be required in a unexploited field before taking up an area for coal bed methane exploration.

Drilling  The design and procedure for drilling a coal bed methane gas well

must achieve the aims of maintaining well control and preventing formation damage.

 The primary concerns for drilling are overpressure of gas/water

kicks, high permeability which leads to loss of circulation fluid, formation damage due to the nature of coal and hole sloughing.

 The rigs commonly used are portable, self-propelled and

hydraulically-driven, having a top head drive. A major problem during drilling could be the excessive water flow. Drilling with pressure may be hindered due to escape of large quantities of water through the coal seams.

CBM Resource Scenario in India  Total coal resource: 248 billion tons

1. Gondwana Basins contribute about 99% of it 2. Damodar Valley Coalfields contribute 50% of

this resource –primary target for CBM Exploration. 3. Estimated CBM resource -0.8 to 1.5 TCM (different sources). 4. ONGC’s preliminary assessment indicate 4 Damodar Valley Coalfields viz. Jharia, Bokaro, North Karanpura and Raniganj to be most prospective.

CBM Potential of India  India is among the top ten countries in coal resources, having an

estimated coal reserve of 160 million metric tons, with an estimated methane resource of 850 BCM. The Indian coal is mainly confined to the Permian Gondwana basins and the tertiaries.

 Tertiary coals are widespread in Assam, Meghalaya, Arunachal Pradesh,

Tamil Nadu, Rajasthan and Gujarat. Tertiary coals are generally found to be lignitic to sub-bituminous in rank and are generally considered to be unsuitable for coal bed methane target.

 However, tertiary coals in petroliferous basins of Cambay, Upper

Assam and Assam–Arakan may be prospective due to reported higher gas content, which is probably stored in the coal after generation from deeper-lying hydrocarbon source beds or may be of biogenic origin.

What CBM Research Says?  Methane emission studies from working mines of

India reported most of the degree three gassy mines (> 10 cubic m/ton), are confined in the four Damodar Valley coal fields, viz. Raniganj, Jharia, Bokaro and North Karanpura in Bihar and West Bengal.  In these areas, the thickest bituminous coals are extensively

developed in the Barakar measurers and in Raniganj measures of Lower and Upper Permian age, respectively.

Conti……….!  The Barakar coal seams are superior to Raniganj coal seams as coal

bed methane targets. Based on thickness and burial depth, rank and quality of coal has the greatest coal bed methane potential in India.

 In India, the Reliance Gas has carried out comprehensive

geologic assessment of coal/lignite basins based on which about 20,000 km 2 of area has been identified as prospective for CBM with estimated in place resource of about 2000 billion cubic metres.



Conti……….!  The recoverable reserve of about 800 billion cubic metres

and gas production potential of about 105 million metre cum per day over a period of 20 years has been estimated.  CBM potential is thus about 1.5 times the present natural gas

production in India, which is capable of generating about 19000 MW of electricity. The potential of gas production in India is given in Table .

World scenario  According to the USGS in 2000, the US CBM resource is about

700 trillion cubic feet (tcf), of which they estimated about 100 tcf to be economic. The US consumes about 22 tcf per year, so CBM presents the equivalent of at least a 5 year supply to our country.  World-wide estimates of CBM resources range from 6,000 to 24,000 tcf, with the former Soviet Union estimated to have two thirds of the resource. China’s resource is estimated at about 1,000 tcf.

Worldwide Interest  At the moment, almost all of the world’s 7.6 bcm/y production of

coal bed methane comes from the US3, where reserves of coal bed methane are conservatively estimated at a third of natural gas reserves. Drilling is concentrated in two areas, Alabama’s Black warrior, where the coal seams are particularly gas-rich; and New Mexico’s San Juan basin, which is tied into the natural gas grid4.

 Australia is likely to be the next producer of commercial

quantities of methane: a couple of pilot projects in Queensland and New South Wales are already well advanced. However, one drawback in such a huge but sparsely populated territory is finding a market for the gas.

Worldwide Interest  China, the world’s largest coal producer, is sitting on more than 700 tcf

(Trillion Cubic feet)of coal bed methane.

 The country already has a fairly widespread system of methane drainage

and capture, to keep its rather gassy mine safe enough to work in.

 But although 60% of drained methane is utilized, most of it is too dilute

to be used in anything other than local power plants or factory sites. The smaller coal producers of the world are doing their bit for coal bed methane too.

 However, the mature industries and crowded territories of most of them

will keep developments modest. Interest around the world is quickening, and world coal bed methane output is on the first track.