Assessing CO2 storage potential in unconventional gas reservoirs is key to proactive policy making for CO2 storage in India

India had the second largest incremental growth in greenhouse gas (GHG) emissions during 1990-2010, adding about 1.3 Gt CO₂-eq. This was associated with large scale economic growth and population increase. GHG emissions in India are projected to increase to 6-7 Gt-CO₂ by the end of 2050 under the current policy scenario, making it an important player in climate mitigation strategies along with USA and China. Unconventional gas reservoirs - namely, unmineable coal seams and shale plays - are exceedingly crucial for effective policy making pertaining to CCS in India. This is so because they incentivize such operations owing to additional revenue generation from incremental gas recovery, which is important since India's conventional gas reserves are very limited. Previous estimates (even though theoretical) on storage capacity in coal seams and depleted oil/gas wells were estimated to be 0.345-5 Gt CO₂ and 3.7-4.6 Gt CO₂ respectively. The current piece addresses the need for assessing the CO₂ storage potential estimates in unconventional gas reservoirs in India, in a step-wise manner. It discusses the possible reasons for deviating from the conservative figures of CO₂ storage capacity which may influence some of the major policy decisions. We estimate the total theoretical CO₂ storage capacity for all combined coal basins to be 677 Gt-CO₂ (480-855 Gt-CO₂). This storage capacity is shared between storage in non-coking coal ( 90%) and coking coal ( 10%) in coal seams of depth >300. CO₂ storage in coal seams of depth <300 m are not considered as these are projected to be used as mineable coal. We also suggest a methodological framework for storage capacity estimation in shale reservoirs based on shale reserves, total organic content (TOC) and CO₂ adsorption capacity characteristic of various shale types. To our knowledge, such an exercise has not been undertaken for Indian shale reservoirs. Furthermore, we connect the theoretical capacity estimates with practical considerations such as geological and technical constraints with storage technologies and source-sink connectivity to narrow down into the realistic and viable capacities in a systematic manner. Some of the important factors at play here involve exploring swelling behaviour of Indian coals and kinetics of gas adsorption to arrive at accurate efficiency factors.