**5. CO2 sequestration**

Since the industrial revolution, concentration of CO2 and other greenhouse gases increased due to burning of fossil fuels. The measured atmospheric concentrations of CO2 are 100 ppm higher than preindustrial levels [46]. According to report published by the Global Monitoring Division (formerly CMDL) of the National Oceanic and Atmospheric Administration, concentration of CO2 in the atmosphere has increased by 26% from less than 320 ppm in 1960 to 405 ppm in 2017 [47]. Fossil

## *CO2-EOR/Sequestration: Current Trends and Future Horizons DOI: http://dx.doi.org/10.5772/intechopen.89540*

fuels provide 86% of the world's energy and this amount is responsible for around 91% of CO2 emissions. There is almost a global agreement about taking responsible actions in implementing energy sources in the years to come [48].

Subsurface CO2 sequestration is one of the various options for reducing the CO2 concentration in the atmosphere resulting from anthropogenic activities. CO2 sequestration potentially provides around 25% of the required mitigation to global emissions, which can delay global warming to an acceptable extent [48]. Subsurface sequestration captures CO2 at the point of its emission and injects large amounts of it deep into subsurface formations where it can be stored permanently [49].
