Introduction
The climate of planet Earth though unstable remained more or less congenial during human history leading to the spread and development of human civilization. With the end of ice age about 10000 years back, mankind developed agriculture and industry generally in a global climate that was warm, pleasant, and mostly predictable. Though regional climates have changed, but by and large it remained within the limit of tolerance.

During last two centuries, for sustaining the global industrial revolution, vast quantities of fossil fuels are being burnt starting with coal and followed by petroleum fuels (oil and gas) to power the developing world. This has resulted in the release of huge amount of CO2 trapped in these fuels into the atmosphere; increasing the atmospheric concentration of CO2 (which has a significant property of absorbing heat) thus creating what is called a Greenhouse Effect and consequent Global Warming.

In this context, the UN Framework Convention on Climate Change (UNFCCC, 1992), which has been ratified by majority of nations, asserts that the world should achieve an atmospheric concentration of greenhouse gases that would avoid “dangerous anthropogenic interference with the climate system.

Greenhouse Gas emissions and Global Warming
Naturally occurring Greenhouse Gases (GHG) in the atmosphere include water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Certain human activities, however, add to the levels of most of these naturally occurring gases. Carbon dioxide is released by burning solid waste, fossil fuels (oil, natural gas, and coal), wood and wood products.  Methane is emitted during the production and transport of coal, oil and natural gas, and also from the decomposition of organic wastes, landfills, and the raising of livestock. Nitrous oxide is emitted during agricultural and industrial activities, as well as during combustion of solid waste and fossil fuels. The modern greenhouse gases like Hydro fluorocarbons (HFCs), Per fluorocarbons (PFCs), and Sulfur hexafluoride (SF6) are generated in a variety of    industrial processes. Each GHG differs in its ability to absorb heat in the atmosphere. HFCs and PFCs are the most heat-absorbent. Methane traps over 21 times more heat than CO2. Nitrous oxide absorbs 270 times more heat than CO2.

Energy from the sun heats the earth's surface; in turn, the earth radiates energy back into atmosphere. Atmospheric GHG trap some of the outgoing energy, retaining heat somewhat like the glass panels of a greenhouse. Without this natural "greenhouse effect," temperatures would be much lower than they are now, and life as known today would not have existed.  Problems arise when the atmospheric concentration of GHG increases.
 

Atmospheric concentrations of CO2 have increased from about 275 parts per million (ppm) in the pre-industrial level (early 1700s) to about 365 ppm today. Methane has doubled & nitrous oxide has risen by about 15%.  These increases have enhanced the heat-trapping capability of the earth's atmosphere.  The major sources of CO2 due to human activities include fossil fuel combustion (65%) and the modification of natural plant ecosystems (35%).  Plant respiration and decomposition of organic matter release CO2 more than 10 times than released by human activities; these releases have generally been in balance     up to the industrial revolution with CO2 absorbed by terrestrial vegetation & oceans. The component wise growth of GHG emissions over two centuries is shown below.

Table 1: Increasing abundance of Greenhouse Gases (1750 to 1999)
GHG	Unit	1750	1999
CO2	PPM	280	367
Methane	PPB	700	1745
N2O	PPB	270	314
Perfluromethane (CF4)	PPT	40	80
Perfluroethane (C2F6)	PPT	0	3
Sulfur Hexafluoride (SF6)	PPT	0	4.2
HFC – 23 (CHF3)	PPT	0	14
CFC – 12 (CF2C12¬)	PPT	0	533

As a result of increasing GHGs, average surface temperature of the earth has increased by 0.6oC over 20th century, worldwide precipitation has increased by about 1%, ice cover has decreased and average sea level has increased.


It is predicted that with current trend of emissions of greenhouse gases, the average global surface temperature may rise 1.4 to 5.8oC in next century leading to grave consequences to the earth and mankind. The top 20 countries w.r.t CO2 emissions in 2001 are depicted below.

Table 2: Greenhouse Gas Emissions – Top 20 countries
Sl.  No.	Country	Total CO2 (MMt)	Per Capita (Tonnes of CO2)	Intensity (Tonnes of CO2/Person) (Sq. Km.)
1	USA	5739	19.8	596
2.	China	3050	2.36	318
3.	Russia	1614	11.16	95
4.	Japan	1158	9.1	3065
5.	India	922	0.88	280
6.	Germany	819	9.93	2294
7.	Canada	573	17.8	57
8.	U.K.	566	9.42	2312
9.	Italy	445	7.67	1477
10.	S.Korea	443	9.17	4498
11.	France	396	6.58	724
12.	S.Africa	386	9.02	316
13.	Australia Ukraine	363	18.39	47
14.	Ukraine	354	7.36	586
15.	Mexico	352	3.3	178
16.	Brazil	351	1.93	41
17.	Iran	330	4.83	200
18.	S.Arabia	310	4.83	158
19.	Spain	303	7.53	600
20.	Poland	288	7.45	921
	Other	5320
	Total World	24082	3.81	163

Indian CO2, CH4 and N2 emissions are less than 3% of global GHG emissions in 2000. Coal and oil product contribute over 60% and Agriculture including Livestock contributes about 29%. PFC, HFC, SF6 contribute less than 1% (globally – 1.5). The current growth rate of emissions in India is 5%. Emissions are predominantly from urban areas though 70% live in village.
In India, it is estimated that GHG emissions will rise to 3% annually over 2030 and will be 5% of global emissions in 2030. Share of per capita emissions will continue to be low due to high population growth rate. CO2 from fossil fuel will remain as main contributor with 2/3 to 3/4th contributions in 2030. The component wise growth is shown in following table.

Mitigation of Climate change
There are basically two options for reducing CO2 concentration in the atmosphere, which include reduction of emissions from the sources on the one hand and sequestering the emitted CO2 on the other. Carbon Dioxide sequestration refers to capturing CO2 and transporting it to a   permanent storage site in a variety of underground geological locales of sufficient capacity. Each of these options will have a role to play in tackling CO2 emissions. The extent to which each technique is used will depend on many factors including costs, potential capacity, the extent to which emissions must be reduced, environmental impacts, rates at which the technology can be introduced and social factors such as public acceptance.

Geological Storage of Carbon Dioxide
The idea of CO2 sequestration in underground geological formations as a mitigation option was conceived in early 1990s, with the understanding of the fundamental concepts and identification of possible geological locales best suited for CO2 sequestration. In 1996, the world’s first large-scale CO2 storage project was initiated by Statoil, Norway and its partners at the Sleipner gas field in the North Sea. By the late 1990’s, a series of major programs were initiated in the United States, Canada, Japan, Europe and Australia. 

As the concept of geological sequestration developed, it was recognized that CO2 can be sequestered in geological formations by three principal mechanism viz. hydrodynamic trapping, solubility trapping and mineral trapping As part of hydrodynamic trapping CO2 can be trapped as a gas or supercritical fluids under low-permeability caprock the way natural gas is trapped in gas reservoir or that gas is stored in aquifer gas storage. Solubility trapping may occur in oil reservoirs through swelling effect which provides the basis of Enhanced oil recovery (EOR). Adsorption of CO2 in coal is an example of Mineral trapping. Keeping the above mechanism in view, three principal types of widespread geological formations have the potential to sequester significant and large amounts of CO2.  They include active, depleted and also abandoned oil and gas reservoirs, deep saline aquifers and deep coal seams.  Other underground geologic formations such as fractured rocks, basalt, oil shale, caverns and mines etc may also provide site specific opportunities but are not likely to be developed in immediate future due to technical and economic factors.  

Several worldwide and national assessment of the storage volume available for sequestration demonstrates the significant potential for geologic sequestration of CO2. Worldwide estimate range from 370 to 11000 Giga tonne (GT) of CO2. According to IEA Greenhouse R&D programme, oil and gas field, deep saline aquifers and unminable coal seams can store CO2 of the order of 920, 400-10000 and 15 GT respectively. However these global storage estimates developed using general assumptions must be considered as the theoretical potential or resource. The storage capacities that can be achieved in actual practice i.e. reserve will potentially differ from these estimates. To determine the actual or realizable potentials more in depth analysis with geographic spread of underground geological storage system is a prerequisite. 
 


Current Global Geological Storage Projects
Currently, the actual or planned commercial CO2 geological storage locations are major high CO2 gas production facilities such as Sleipner and Snohvit gas fields in the North Sea, In Salah in Algeria and Gorgon in Australia. In view of the opportunities offered by enhanced oil recovery (EOR) operations, there is now interest in the storage potential of EOR with associated economic benefits and some are driven by a carbon tax regime. The three largest CO2-EOR projects in the United States are the SACROC and the Wasson-Denver in the Permian basin in Texas, and the Rangely Weber project in the Rocky Mountain of Colorado. Smaller numbers of CO2-EOR projects are also currently underway in Argentina, Trinidad, Turkey and Canada. In Canada, a CO2¬-EOR project has been established by EnCana at the Weyburn oil field in Southern Saskatchewan. To date, there has been only one CO2 enhanced CBM recovery (ECBM) demonstration project, located in the northern San Juan Basin of north-central New Mexico, United States.