India’s Hydrocarbon Outlook 2024 173 A Report on Exploration & Production Activities other pollutants. It is extracted by reducing the pressure within the coal seams through water pumping, which releases the adsorbed methane gas. There are four types of terms used to describe the methane emanating from coal beds: a. Coalbed Methane (CBM) or Coal Seam Gas (CSG) - A generic term for themethanerich gas occuring naturally in coal seams typically comprising 80% to 90% methane with lower proportions of ethane, propane, nitrogen, and carbon dioxide. This refers to the methane recovered from unmined coal seams using surface boreholes. b. Coal Mine Methane (CMM) - Methane gas captured at working mine by underground methanedrainage techniques. This includes gas captured underground, whether drained in advance or after mining, and any gas drained from the surface of well. c. Ventilation Air Methane (VAM) - Methane emitted from coal seams that enters the ventilation air and are exhausted from the ventilation shaft at a low concentration, typically in the range of 0.1% to 1.0% by volume. d. Abandoned Mine Methane (AMM) - The methane gas recovered from abandoned coal mines. 1.2 Shale Gas and Shale Oil: Gas or oil within shales are the un-expelled part during petroleum generation process and found trapped within the pore space of shale as the source rock. Genetically, shales have no intrinsic permeability for self-flow to a well bore. Like all other unconventional resources of continuous nature, shale plays too have less geological risks than conventional but more development risks. Shale gas and oil have significantly increased global energy supplies and have transformed the energy landscape in countries like the USA. The commercial extraction of shale gas and oil involves the use of horizontal drilling followed by hydraulic fracturing (fracking). These techniques open up more reservoir contact and create sufficient fracture conductivity to release the trapped gas and oil. 1.3 Tight Gas: Tight gas refers to natural gas trapped in low-permeability rocks other than shales, viz. sandstones, siltstones or carbonates. Like shale gas, tight gas extraction requires hydraulic fracturing to create flow conduits. Tight gas is genetically conventional deposits, however due to location in the deeper part of the basins, it loses intra-granular porosity due to excessive over-burden pressure. For the same reasons, such deposits are often developed into high pressure and high temperature conditions making gas extraction more challenging. 1.4 Gas Hydrates: Gas hydrates are crystalline ice-like structures that contain methane and other gases. They are found in deep ocean several hundreds meter below seabed and also in permafrost regions. It is formed under moderately high pressure but low temperature conditions. Extraction is still experimental, involving methods like depressurization, thermal stimulation, and inhibitor injection to dissociate the hydrate. Gas hydrates hold vast amount of natural gas as 1-unit hydrate volume expands into ~160 unit gas to surface and represent a potential future energy source. 1.5 Oil shales: Oil shales consist of a mixture of shales, sands, water, clay, and bitumen, a heavy viscous form of crude oil. It is genetically immature source rocks formed due to failed process of petroleum generation. Oil is extracted by surface mining and in-situ techniques, such as steam-assisted gravity drainage. Oil shales are a significant source of heavy oil, but their development raises environmental concerns due to greenhouse gas emissions and land disturbance.
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