Compositional modeling to analyze the effect of CH₄ on coupled carbon storage and enhanced oil recovery process

The present study is aimed at the development of compositional simulation models of the co-injection of CO₂ and CH₄ during the water-alternating-gas (WAG) process in order to assess the efficiency of carbon capture and storage in combination with enhanced oil recovery (CCS-EOR). The co-injection of CO₂ and CH₄ occupies more reservoir pore volume and causes higher reservoir pressure than CO₂ WAG, thus leading to an enhanced early EOR performance. However, the overall EOR performance of the co-injection method becomes lower than that of CO₂ WAG due to the reduced miscibility and sweep efficiency upon further CH₄ addition. The decrease in gas displacement and sweep efficiency weaken the hysteresis effects upon the residual trapping mechanism. However, the solubility trapping mechanism takes effect because the co-injection generates higher average reservoir pressure than does the CO₂ WAG. The index of global warming potential (GWP) in a mole unit is employed to quantify the carbon storage effects of CO₂ and co-injectionWAG cases. According to the index, 1 mole of CH₄ sequestration has the same effects as that of 10 moles of CO₂ for global warming mitigation. In conclusion, the carbon storage effects are enhanced as CH₄ concentration in the WAG increases.