Experimental study on the effect of decomposition induction on the replacement process of CO2[sbnd]CH4 hydrate in hydrate-bearing sediments below the freezing point

Abstract

The replacement technique of CH4 by CO2 is a promising strategy for natural gas hydrate exploitation, which has been regarded as a win-win situation for simultaneously achieving CO2 storage and CH4 recovery in permafrost region. How to improve the replacement efficiency is a fundamental issue related to the natural gas hydrate exploitation in permafrost region. In this work, the replacement process of CO2CH4 hydrate was carried out in the ice-sand mixture system and the replacement characteristics were further illustrated through experiment. The influence of different decomposition induction on the replacement process CO2CH4 hydrate was deeply analyzed, especially for the decompression driving force (ΔP) and thermal driving force (ΔT). The results showed that the ΔP has a significant inductive effect on the replacement process of CO2CH4 hydrate in porous media below the freezing point. The replacement efficiency attained 12.25% under condition of decomposition induction driven by ΔP with -0.1 MPa, which was raised 3.09% in comparison. However, the larger ΔP didn't mean that it was more favorable to the replacement process of CO2CH4 hydrate, and there was an optimal value in the experiment. Furthermore, an additional thermal drive (ΔT) is beneficial to the replacement efficiency of CH4 hydrate and the sequestration efficiency of CO2. It greatly improved the CH4 recovery and CO2 sequestration ratio during the replacement process of CO2CH4 hydrate. The replacement efficiency attained 20.34% under condition of depressurization drive combined with thermal drive. It was worth noting that the contribution of thermal drive (ΔT) was different on CH4 recovery and CO2 sequestration. There was an optimal value for the enhancement on the replacement process of CO2CH4 hydrate. These results are of significance for providing a theoretical guidance for natural gas hydrate exploitation in permafrost regions.