Predictions on CH4 recovery factors using the CO2 replacement method to develop natural gas hydrate resources

Abstract

Injecting CO2 into the hydrate reservoirs to replace CH4 is an effective method for the natural gas hydrate (NGH) exploitation and CO2 sequestration. The accurate prediction of the CH4 recovery factor under different pressures, temperatures, and CO2 injection amounts are essential for the design of the CO2 replacement process.However, conventional multiphase flash algorithms are not stable for CO2-containing hydrate system calculations because of inappropriate initial estimations on the hydrate phase fraction and composition. This paper proposed a new multiphase flash algorithm by developing a new initialization method for the hydrate phase fraction and composition as well as the solution procedure. A total of 61 sets of experimental data at pressures from 8 MPa and 16 MPa are applied to validate the new method. Results demonstrate that the new method successes in all the 61 cases, while the software PVTsim and CSMGem do not converge for 16 and 20 cases, respectively. Also, the average relative deviation between the calculated phase compositions and experimental values is less than 18 %, which represents a comparable accuracy to the above two software. Moreover, case studies are designed to research the variations of the CH4 recovery factor with the injected CO2 concentration in the hydrate reservoir. The most economical injected CO2 concentrations are discussed at specified reservoir conditions.