With increasing worldwide CO2 emissions, interest has risen in injecting CO2 into coals to mitigate their possible role in global warming.Concepts from natural gas storage are readily adapted to CO2 sequestration in coals.The classic p/Z plot for describing stored gas inventories is modified for CO2 sequestration using the p/Z* concept developed for understanding conventional coal gas recovery.Coal often exhibits stress dependent permeability and this relationship is used to identify the optimal depth for CO2 injection.CO2 fillup of a coal seam is described by coupling a gas injectivity equation with a mass balance equation.Example calculations are presented for two selected basins.
Underground storage of natural gas has been practiced for nearly a century.Depleted gas reservoirs are prime candidates for this purpose but depleted oil fields and aquifers have also been used for gas storage.The major difference between underground gas storage and CO2 sequestration in coals is that in the former the gas will be withdrawn, often during the course of the next heating season, while in the latter the goal is permanent immobilization of the CO2.Despite this difference, several engineering concepts from underground storage of natural gas apply directly to CO2 isolation in coals.The three engineering objectives of natural gas storage articulated by Katz and Tek are readily adapted for CO2 sequestration:
1. Verification of inventory.For a given coal deposit, we need to know the volume of injected CO2 and how it relates to ultimate storage capacity.This is can be accomplished by monitoring reservoir pressure.
2. Retention against migration.A monitoring system is required to assure that CO2 is not migrating out of the coal deposit.CO2 can escape from a coalbed either as fugitive gas or dissolved in mobile groundwater.A decline in reservoir pressure would indicate loss of CO2 while a system of surveillance wells to monitor gas and water composition would help identify migration pathways.
3. Gas injectivity.Deliverability of gas, when the storage reservoir is depleted at the end of the heating season, leads to the requirement in CO2 sequestration that the injectivity of wells as the coal deposit approaches fillup is critical.
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