Evaluation of Different CH₄-CO₂ Replacement Processes in Hydrate-Bearing Sediments by Measuring P-Wave Velocity

The replacement of methane with carbon dioxide in natural gas hydrate-bearing sediments is considered a promising technology for simultaneously recovering natural gas and entrapping CO₂. During the CH₄-CO₂ replacement process, the variations of geophysical property of the hydrate reservoir need to be adequately known. Since the acoustic wave velocity is an important geophysical property, in this work, the variations of P-wave velocity of hydrate-bearing sediments were measured during different CH₄-CO₂ replacement processes using pure gaseous CO₂ and CO₂/N₂ gas mixtures. Our experimental results show that P-wave velocity continually decreased during all replacement processes. Compared with injecting pure gaseous CO₂, injection of CO₂/N₂ mixture can promote the replacement process, however, it is found that the sediment experiences a loss of stiffness during the replacement process, especially when using CO₂/N₂ gas mixtures.