CO2/N2 mixture sequestration in depleted natural gas hydrate reservoirs

Abstract

Carbon dioxide (CO2) emission is one of the primary causes of global warming, while CO2 storage is an effective way to solve this problem. Moreover, CO2 storage as CO2 hydrate is regarded as a viable and promising way. The development of natural gas hydrate mining technologies has provided a safe and appropriate method to store CO2 in depleted gas hydrate reservoirs. In addition, different concentrations of N2 are contained in the captured CO2 from flue gas. Therefore, CO2 storage with different N2 concentrations in depleted natural gas hydrate reservoirs with excess water was investigated in this study. Three types of gas (pure CO2, 90 mol%CO2 + 10 mol%N2 and 80 mol%CO2 + 20 mol%N2) and three pressures (3.0 MPa, 3.2 MPa and 3.5 MPa) in the constant pressure process were investigated. Magnetic Resonance Imaging (MRI) was employed to monitor the entire experimental process. The experimental results showed that the hydrate was nucleated and generated in different areas independently during the storage process. N2 was beneficial for quick hydrate formation; however, pure CO2 was most efficient for hydrate formation from a long-term perspective. In addition, CO2 was primarily stored as hydrate; a portion of CO2 was stored in the form of gas; and a small amount of CO2 was dissolved in the pore water. Moreover, the CO2 storage density (226.56 kg/m3) was the highest at the conditions of 275.15 K and 3.5 MPa using 90 mol%CO2 + 10 mol%N2 in the experiments. The highest CO2 storage density (229.99 kg/m3) calculated through water conversion occurred at the conditions of 275.15 K and 3.5 MPa using pure CO2.