Experimental Study on the Mechanical Properties of CH4 and CO2 Hydrate Remodeling Cores in Qilian Mountain

The CH4-CO2 replacement method has attracted global attention as a new promising method for methane hydrate exploitation. In the replacement process, the mechanical stabilities of CH4 and CO2 hydrate-bearing sediments have become problems requiring attention. In this paper, considering the hydrate characteristics and burial conditions of hydrate-bearing cores, sediments matrices were formed by a mixture of kaolin clay and quartz sand, and an experimental study was focused on the failure strength of CH4 and CO2 hydrate-bearing sediments under different conditions to verify the mechanical reliability of CH4-CO2 replacement in permafrost-associated natural gas deposits. A series of triaxial shear tests were conducted on the CH4 and CO2 hydrate-bearing sediments under temperatures of −20, −10, and −5 °C, confining pressures of 2.5, 3.75, 5, 7.5, and 10 MPa, and a strain rate of 1.0 mm/min. The results indicated that the failure strength of the CO2 hydrate-bearing sediments was higher than that of the CH4 hydrate-bearing sediments under different confining pressures and temperatures; the failure strength of the CH4 and CO2 hydrate-bearing sediments increased with an increase in confining pressure at a low confining pressure state. Besides that, the failure strength of all hydrate-bearing sediments decreased with an increase in temperature; all the failure strengths of the CO2 hydrate-bearing sediments were higher than those of the CH4 hydrate-bearing sediments in different sediment matrices. The experiments proved that the hydrate-bearing sediments would be more stable than that before CH4-CO2 replacement.