Molecular simulation study on carbon dioxide replacement in methane hydrate near the freezing point

Abstract

Carbon dioxide (CO2) replacement method for hydrate extraction offers advantages such as resource utilization, greenhouse gas reduction, carbon sequestration, geological stability and controllability. The enhancement of CO2 replacement efficiency for methane hydrates and the achievement of more efficient hydrate extraction have consistently been the primary focus of researchers. In this work, we explored the CO2 replacement process in methane hydrates at different temperatures above the methane hydrate phase equilibrium line using molecular simulations. The results demonstrate that the replacement process of CO2 for methane hydrates occurred spontaneously even within the equilibrium region of methane hydrates. The CO2 replacement of methane molecules in hydrates can be divided into three steps: intrusion of CO2 molecules into the semi-cage structures, methane hydrates decomposition and CO2 hydrates formation. The decomposition of methane hydrates plays a crucial role in determining the replacement process. Moreover, the replacement process was significantly affected by temperature, and CO2 showed the highest replacement efficiency at 272 K. At this temperature, CO2 molecules exhibited higher activity, rendering them more likely to disrupt the pentagonal or hexagonal ring structures of the methane hydrate cages. Our simulation results provide microscopic insights into the carbon dioxide replacement in methane hydrate below and above the freezing point.