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.

将CO ₂注入水合物储层以替代CH ₄是天然气水合物（NGH）开采和CO ₂固存的有效方法。在不同压力，温度和CO ₂注入量下，CH ₄回收率的准确预测对于设计CO ₂替代工艺至关重要。

但是，常规的多相闪蒸算法对于CO ₂不稳定含水合物系统的计算，因为对水合物相分数和组成的初步估算不当。通过开发一种新的水合物相组成和组成初始化方法以及求解步骤，提出了一种新的多相闪蒸算法。在压力为8 MPa和16 MPa的情况下，总共使用了61组实验数据来验证该新方法。结果表明，该新方法在全部61例病例中均成功，而软件PVTsim和CSMGem分别在16例和20例病例中未融合。而且，计算出的相组成和实验值之间的平均相对偏差小于18％，这代表了与上述两个软件相当的精度。此外，案例研究旨在研究CH的变化水合物储层中注入的CO ₂浓度与₄采收率有关。在指定的储层条件下讨论了最经济的注入CO ₂浓度。