COAL BED METHANE (CBM) RECOVERY

It is well known that coal is framed due to bio conversion of fossilised organic matter, In the process of coal formation, anaerobic conditions led to generation and trapping of methane in this coal seams. The pressure exerted by naturally formed water keeps the methane "absorbed" on internal surfaces of coal. Thus, coal bed gas is in mono-molecular state and not as free gas, as in natural oil/gas fields. Therefore, all coal fields of the world have coal bed methane, the only difference being the quantity of gas in individual coal seams.

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 (microporosity). Microporosity 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.

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.

The generation of methane gas results from high temperature and pressure due to continuous burial. During the transformation process, coal becomes rich in carbon and large amount of fluid matter is released like methane, carbon dioxide and water. Such generation of fluid is significant in bituminous and higher rank coal with maximum yield of 150-200 cm³ per gram of coal. Indian's coals have gas content values ranging from 1 to 23m³/tonne.

In India, the Reliance Gas has carried out comprehensive geologic assessment of coal/lignite basins based on which about 20,000 km² of area has been identified as prospective for CBM with estimated in place resource of about 2000 billion cubic metres. 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 9.

Table 9 : CBM production potential in India

(Source : Coalbed Methane : A Survey by Reliance Gas (P) Limited)