Relationship between Creep Property and Loading-Rate Dependence of Strength of Artificial Methane-Hydrate-Bearing Toyoura Sand under Triaxial Compression

Abstract

Abstract:Methane hydrate is anticipated to be a promising energy resource. It is essential to consider the mechanical properties of a methane hydrate reservoir to ensure sustainable production, since its mechanical behavior may affect the integrity of the production well, the occurrence of geohazards, and gas productivity. In particular, the creep property of methane-hydrate-bearing sediment is thought to have great significance in the long-term prediction of the mechanical behaviors of a reservoir. In earlier studies, triaxial compression tests were conducted on artificial methane-hydrate-bearing Toyoura sand under three axial-loading conditions, i.e., constant-strain-rate test, constant-stress-rate test, and creep (constant-stress) test. In this paper, the time-dependent properties of the methane-hydrate-bearing Toyoura sand observed in these tests were quantitatively discussed and found to be almost in agreement. The creep life obtained from the creep tests had a reasonably strong correlation with the loading-rate dependencies of strength, obtained from the constant-strain-rate tests and constant-stress-rate tests based on a simple hypothesis. The findings are expected to be used to develop a constitutive model considering the time-dependent behaviors of hydrate-bearing soil in future studies, and to improve the reliability of long-term prediction of the geomechanical response to gas extraction from a reservoir.

Strain Rate‐Dependent Hardening‐Softening Characteristics of Gas Hydrate‐Bearing Sediments

“…In agreement with earlier studies (Miyazaki et al, ), we hypothesize that the alteration of GH‐sediment structures during plastic deformation can partly be assigned to creep and viscous deformation of GH. In contrast to our results with gas‐saturated samples, former studies with water‐saturated samples indicate higher peak strength values at faster strain rates (Miyazaki et al, ), which suggests that strain‐induced alteration of GH structures in water‐saturated sediments is different from gas‐saturated systems.…”

“…Time-and rate-dependent effects on the stress-strain behavior of GHBS have received less attention in the past. However, viscous deformation and creep strain of GHBS were reported to be relevant (Miyazaki et al, 2010) and similar to frozen sand (Miyazaki et al, 2017). Experiments on pure GH have shown high stiffness, strength, and ductility, as well as the relevance of strain-induced structural alterations (Durham et al, 2003;Jia et al, 2016;Stern et al, 1996).…”

Creep Behaviors of Methane Hydrate-Bearing Frozen Sediments

“…When the damage enhancement and healing offset each other, the strain rate eventually remain constant. Similar creep behaviors of the sediments were discovered by Miyazaki et al [20,27].…”

“…This is because there are more initial micro cracks in the clay-made specimen, since the filling effect of ice and hydrate should be generated in the primary stage. When the axial load is not high enough, along with the elapsed time, some micro cracks would generate, propagate, and unite, the creep of the soil skeleton and the visco-plastic flow of hydrate and ice would slow down correspondingly, and eventually the unstable cracks would propagate [14,27].…”

“…Higher deviator stress, which responds to higher external load, generates more micro fractures in the specimen, weakens the skeleton structure, and deforms the specimen more. Much of the literature also observed a larger creep strain rate at higher deviator stress for frozen soil, pure methane hydrate, and THF/methane hydrate-sand specimens [16,19,[27][28][29]. Moreover, the relationship between strain rate and elapsed time in the logarithmic coordinate system is also shown in Figure 3, where the slope approaches a constant value during creeping, and enhances with the increase of deviator stress, which means the strain rate of specimens under higher deviator stress was reduced more slowly.…”