Mechanism of CO2 injection to enhance gas recovery and carbon storage in depleted bottom-water gas reservoirs
CO2 injection in gas reservoirs to enhance gas recovery and carbon storage is a potential key technology to significantly improve gas production and reduce carbon emission.In order to provide a guidance for the efficient development of bottom-water gas reservoirs,this paper studies the influence of formation water salinity on the phase equilibrium of the CO2-H2O-CH4-salt system,the influence of pressure change on the phase equilibrium of the CO2-H2O-CH4-salt system in the process of injecting CO2 into the gas reservoir,the influence of injection and production scheme on CO2 EOR,and the influence of salinity on CO2 EOR and storage.And the following research results are obtained.First,with the increase of salinity,the dissolubility of CO2 and CH4 in salt water decreases,and the density and viscosity of liquid phase increase.Salinity has little influence on the properties of gas phase.Second,with the increase of pressure,the dissolubility of CO2 and CH4 in liquid phase,the density and viscosity of gas and liquid phases,and the deviation factor of liquid phase increase,while the deviation factor of gas phase decreases first and then increases.Third,in the case of simultaneous injection and production,CH4 production rate is stable,and less CO2 is produced.In the case of injection first and then production,the mixing of CO2 and CH4 is accelerated,and the amount of CO2 storage is small.It is indicated that the former case is more suitable for CO2 EOR and storage.Fourth,in the precondition without considering the salting-out effect,the influence of salinity on CH4 recovery factor and CO2 storage is nearly negligible.No matter what salinity of the depleted bottom-water gas reservoir is,the CH4 recovery factor is over 80%,and the CO2 storage rate is over 99%.In the process of short-term CO2 injection,CO2 is stored mainly in the form of gaseous or supercritical state,with a little dissolved in liquid.And the mass fraction of CO2 dissolved in liquid after 100 years is about 5%.In conclusion,CO2 injection in depleted bottom-water gas reservoirs can enhance gas recovery and realize CO2 storage.
Depleted bottom-water gas reservoirCO2 injectionEnhanced gas recoveryCarbon storagee-CPA equation of stateCO2-CH4-H2O-salt systemPhase transition law
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