Applied Energy, Vol.236, 825-836, 2019
Efficiency enhancements in methane recovery from natural gas hydrates using injection of CO2/N-2 gas mixture simulating in-situ combustion
Thermal stimulation was combined with an injection of a mixture of CO2 (85%) + N-2(15%) to investigate efficiency enhancements from pure thermal stimulation and thermal stimulation with CO2 injection approaches. Tests were performed at initial hydrate saturation of 10% and 300 ml/min CO2 + N-2 injection rate with three different heating rates of 20, 50 and 100 W. The results indicate that thermal stimulation with CO2 + N-2 injection is the most efficient method available for methane gas recovery. At 10% Hydrate Saturation (SH) and 100 W heating rate, the number of moles of CH4 recovered increased from 8.5 to 16 to 20 in the case of thermal stimulation, thermal stimulation with CO2 exchange and thermal stimulation with CO2 + N-2 exchange respectively. The experimental results reported here are aligned with model and Raman spectroscopy predictions in terms of replacement mechanism and recovery efficiency, reported in the literature. The results obtained from CO2/N-2 composition ratio show that in the exchange process, CO2 first replaces CH4 in the large cages of Structure I hydrates followed by N-2 targeting CH4 in the small cages of Structure I hydrates. This replacement mechanism has been predicted in the literature by Liu et al. (2016) using Molecular Dynamics simulations. It is also found from this work that N-2 is selectively captured in hydrate cages below 12 degrees C. The values of carbon sequestration index (defined as moles of CO2 sequestered divided by moles of CH4 recovered) were 0.32, 0.52 and 0.85 respectively for 100, 50 and 20 W heating tests. The data obtained from our work in terms of gas composition, methane recovery and CO2 sequestered is consistent with the key findings reported in the literature.