Microstructure Evolution of Hydrate‐Bearing Sands During Thermal Dissociation and Ensued Impacts on the Mechanical and Seepage Characteristics

Abstract

In this study, a microfocus X‐ray computed tomography‐based triaxial testing apparatus was used to observe and quantify the microstructure evolution of hydrate‐bearing sands during thermal dissociation of hydrate. Three triaxial shear tests with X‐ray computed tomography were conducted to study the influence of hydrate dissociation on the mechanical behavior of hydrate‐bearing sands. The results show that hydrate covering the sand particle surface dissociates first and then at the menisci between sand particles. The secondary hydrate formation mainly occurs on the menisci between sand particles and the surfaces of the hydrate shells where hydrates already exist. Hydrate dissociation could cause fabric changes in hydrate‐bearing sands, resulting in a more isotropic orientation distribution of sand particles. The failure behavior of the hydrate‐free sand specimen is similar to that of the specimen after hydrate dissociation, which shows an obvious drum‐shaped failure pattern with X‐shaped shear bands. However, the hydrate‐bearing sand specimen exhibits a shear band with a determined thickness and inclination angle. Hydrate dissociation could cause a significant loss of supporting cementation, resulting in a decline in the stiffness and failure strength. The failure strength of hydrate‐free sand specimen is slightly higher than that of the specimen after hydrate dissociation; this may due to that the homogeneous orientation frequency distribution can enhance the stability of the force chain between sand particles. The secondary hydrate formation causes sediment deformation resulting in a decrease in absolute permeability due to pore blockage.

Mechanical behaviors of hydrate-bearing sediment with different cementation spatial distributions at microscales

“…Finally, the shear band becomes concentrated with a determined inclination angle and thickness. This is consistent with previous studies on hydratebearing sediments with plane strain shear tests (Thielicke and Stamhuis, 2014;Kato et al, 2016;Kajiyama et al, 2016) and CT-based triaxial shear tests (Wu et al, 2020a;Yoneda et al, 2016).…”

“…In addition, the pore space refers to the pore space without XeH. For the CT images from the full CT scan, the confining fluid, rubber membrane, and metal permeable stones have to be removed using a mask first, and then the thresholds of the different components (the pore space, the sand particles, and the XeH) were determined by repeated comparisons of the volumes of the different components obtained from the CT images to the real volumes ( Wu et al., 2020a ). The specific segmented effect could be found in Figure 9 , and each phase could be reflected effectively.…”

“…Triaxial tests on hydrate-bearing interlayered sediments were also conducted, and it was found that the stress-strain response of hydrate-bearing interlayered sediments is substantially different from that of hydrate-bearing sediments, and mechanical parameters such as the strength, elastic modulus, and initial yielding modulus are deeply affected by the hydrate distribution ( Dong et al., 2019 ; Li et al., 2018 ), illustrating that the hydrate spatial distribution should be taken into consideration when evaluating the geomechanical behavior of hydrate-bearing sediments. With the introduction of X-ray CT, Wu et al. (2020a) studied the failure behavior of hydrate-free zones, cementing hydrate-bearing zones and patchy cluster hydrate-bearing zones in the different stages of the stress-strain curve.…”

Pore‐Scale 3D Morphological Modeling and Physical Characterization of Hydrate‐Bearing Sediment Based on Computed Tomography

“…For the absolute permeability simulation via FVM, an incompressible Navier‐Stokes equation (Chorin, 1997; Harlow & Welch, 1965) was used as in literature (Bird et al, 2014; Wu, Li, Liu, Liu, et al, 2020), and the fluid density and fluid dynamic viscosity were considered constants while the relevant velocities were sufficiently small not produce turbulence: where is the divergence operator, is the gradient operator, is the velocity of the fluid, μ is the viscosity of the flowing fluid, ∇ 2 is the Laplacian operator, and P is the pressure of the fluid.…”

Influence of Water Saturation, Grain Size of Quartz Sand and Hydrate-Former on the Gas Hydrate Formation

“…At the same time, natural gas as a fuel is much more environmentally friendly than coal and oil since it has a high calorific value; when it is burned, only carbon dioxide and water are formed [9,15]. Four main methods of gas hydrate field development have been proposed [16]: depression [17,18], heating [19,20], injection of inhibitors [21][22][23], and replacement with carbon dioxide [24,25]. In this regard, the study of the formation and decomposition of gas hydrates in porous media is a key point for a fundamental understanding of natural gas hydrates' behavior, the development of technologies for their extraction and practical application in the processes of transportation and storage.…”