Integrated Effects of Key Process Variables on CH4–CO2 Exchange in Gas Hydrates: Insights into Driving Forces and Mechanisms

Abstract

Methane (CH4) hydrates are considered a promising low-carbon energy resource due to their vast global reserves; however, their exploitation poses environmental risks, particularly CH4 leakage. The CH4–CO2 exchange process, also referred to as CH4/CO2 replacement, has been proposed as a strategy to mitigate these risks, as it enables simultaneous CH4 recovery and CO2 sequestration while maintaining sediment integrity. Despite extensive investigation, fundamental questions regarding the driving forces and mechanisms governing this process remain unresolved, limiting large-scale implementation. This study evaluates the integrated effects of key process variables (pressure, temperature, reaction time, initial CH4 and CO2 molar amounts, N2 addition, and biosurfactant presence) on the CH4–CO2 exchange process. The proposed methodology enables systematic investigation in a controlled bulk-hydrate system while incorporating selected reservoir-relevant characteristics, such as the presence of excess free water. Furthermore, GC-BID analysis was employed to obtain quantitative gas-phase compositions during the exchange process. The results indicate that the primary driving force for CH4–CO2 exchange is the tendency toward equalization between gas-phase CO2 and hydrate-phase CH4. A phenomenological model was developed to quantify this effect, describing the observed dependence of exchange efficiency on the initial CO2/CH4 molar balance. Mechanistically, the observed behavior is more readily reconciled with a sequential pathway in which partial destabilization of CH4 hydrates facilitates subsequent CO2 incorporation than with direct guest displacement within an intact CH4 hydrate lattice. Elevated temperatures and reduced pressures favored hydrate destabilization and emerged as important enabling factors for exchange performance. Notably, CO2 addition in the absence of destabilization did not promote CH4 recovery, and N2 addition did not improve exchange performance. These findings provide new mechanistic and quantitative insights into CH4–CO2 exchange and contribute to the development of more efficient CH4 hydrate exploitation and CO2 sequestration strategies.