Abstract
The replacement of methane for carbon dioxide in natural gas hydrate-bearing sediments is a promising technology in view of a more sustainable use of fossil fuels. While previous studies have shown successful CH4-CO2 replacement in hydrates, the mechanical response of hydrate-bearing sediments during CO2 injection, CH4-CO2 replacement, and CH4 production needs to be adequately understood in order to avert production problems such as borehole instability, sand production, and buckling of the casing. We take advantage of the characteristics of elastic mechanical wave propagation in sediments to monitor CH4 hydrate-bearing sands before, during, and after CO2 injection. Results show that CH4-CO2 replacement occurs without a loss of stiffness in the granular medium. This implies that CO2-flooded sandy reservoirs can remain mechanically stable during and after CH4 gas production. On the other hand, dry CO2 dissolves hydrate, and continued sediment flushing with dry CO2 reduces the degree of hydrate saturation in the pore space, opens the pore throats, and weakens the granular skeleton. This phenomenon may cause a significant loss of strength near the injection points and regions subjected to high liquid CO2 flow rate. The results of complimentary analyses show a decrease in bulk stiffness as water is displaced by liquid CO2, a stiffening of the granular skeleton during hydrate formation at contacts (diffusion limited), and the implications of water solubility in liquid CO2. (C) 2011 Elsevier Ltd. All rights reserved.