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

Abstract

Abstract:The CH 4 -CO 2 replacement method has attracted global attention as a new promising method for methane hydrate exploitation. In the replacement process, the mechanical stabilities of CH 4 and CO 2 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 CH 4 and CO 2 hydrate-bearing sediments under different conditions to verify the mechanical reliability of CH 4 -CO 2 replacement in permafrost-associated natural gas deposits. A series of triaxial shear tests were conducted on the CH 4 and CO 2 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 CO 2 hydrate-bearing sediments was higher than that of the CH 4 hydrate-bearing sediments under different confining pressures and temperatures; the failure strength of the CH 4 and CO 2 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 CO 2 hydrate-bearing sediments were higher than those of the CH 4 hydrate-bearing sediments in different sediment matrices. The experiments proved that the hydrate-bearing sediments would be more stable than that before CH 4 -CO 2 replacement.

Strength Behaviors of Remolded Hydrate-Bearing Marine Sediments in Different Drilling Depths of the South China Sea

“…From the figure, it is clear that all HBMS display strain hardening on the stress-strain curves, occurring over three stages: the elastic-plastic stage, the yield stage and the critical state stage. The maximum stress of HBMS mostly occurs at approximately 7%-8% strain, which is consistent with previous work [26,38,44,56].…”

“…A comparison between the mechanical properties of HBMS formed by S-1, S-2, S-3 and S-4 marine sediments in Figure 6 clearly shows that the elastic modulus and failure strength of HBMS S-3 and S-4 are higher than those of other HBMS, which can be explained by the fact that S-3 and S-4 marine sediments have a better particle gradation, as shown in Table 1. For the sediment matrix with a better particle gradation, the number of contact points between large particles and fine particles in the unit volume may increase and enhance the shear strength of HBMS [38,41]. However, it is clear that HBMS S-5 displays lower stiffness and failure strength compared to the other HBMS.…”

“…Another reason for this phenomenon is that, as shown by the SEM images of S-5 marine sediments, the soil particles are mostly a sticky flocculent shape. According to Hyodo et al [38], high effective stress may produce the occurrence of a sticky flocculent shape of soil particles, reducing the failure strength of hydrate-bearing sediments. Hence, for this study, the increase of drilling depth would bring about higher effective stress and lead to the decline of the failure strength in HBMS.…”

“…Up to now, a series of experimental studies on artificial hydrate-bearing sediments have been conducted systematically in the laboratory [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35]. It has been proven that the confining pressure, sediment matrix, hydrate saturation and hydrate type have significant influences on the failure strength, stiffness and volume characteristics of hydrate-bearing sediments [9,[36][37][38][39][40][41][42][43][44].…”

A Geomechanical Model for Gas Hydrate Bearing Sediments Incorporating High Dilatancy, Temperature, and Rate Effects

“…As discussed in the introduction, most of the analyses associated with the effect of temperature on MHBS have been based on thermal-dependent nonlinear elastic models (e.g., [23,28]). However, several laboratory investigations (e.g., [6,20,31,[94][95][96]) focused on the behavior of MHBS in the hydrate-stability zone indicate that the thermal effects impact on their mechanical response beyond the elastic domain. In this section we extend the constitutive model presented in Section 2.1 to account for the effect of temperature in the MHBS hydrate stability zones.…”