Insights into recovery of multi-component shale gas by CO2 injection: A molecular perspective

J Zhou and ZH Jin and KH Luo, FUEL, 267, 117247 (2020).

DOI: 10.1016/j.fuel.2020.117247

Understanding the mechanism behind shale gas recovery is of great importance for achieving optimum shale gas productivity. In this work, we use Grand Canonical Monte Carlo (GCMC) simulations to investigate the adsorption and recovery mechanisms of ternary hydrocarbon mixtures comprising methane, ethane and propane in kerogen nanopores. For the adsorption of hydrocarbon mixtures in kerogen slit pores, density distributions of each component are analyzed and the results indicate that densities of methane and ethane in the first adsorption layer increase as pressure increases, while an opposite trend is observed for propane. A stronger confinement effect is observed on the heavier hydrocarbon components, increasing the difficulty of recovery. For the recovery of the multi-component shale gas, we propose a reference recovery route with pressure drawdown and CO2 injection combined and the recovery efficiency is compared to the condition with only pressure drawdown applied. Significant enhancement in recovery ratio for all three components is observed with the CO2 injection and a better performance is shown on heavier components and smaller pores. An increase of 60% and 40% in propane recovery ratio is achieved in the 2-nm and 4-nm kerogen slit pores, respectively. Recovery mechanisms of pressure drawdown and CO2 injection are investigated in detail. The pressure drawdown method recovers methane from the first adsorption layer and middle of slit pore simultaneously, while extracting ethane and propane mainly from the middle of slit pore; the recovery due to CO2 injection mainly takes place in the adsorption layers. Pressure drawdown tends to extract the lighter components and CO2 injection is efficient in the recovery of heavier hydrocarbons. As pore width increases, the recovery ratio of pressure drawdown increases, while that of CO2 injection decreases. Besides, the CO2 sequestration ratio is higher in smaller kerogen slit pores.

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