Mechanical behavior of reservoirs with heterogeneous hydrate distribution: Analysis from the thickness and angle of the hydrate-bearing layer

Abstract

The heterogeneity of natural hydrate-bearing sediments (HBSs) makes it of great engineering significance to investigate the mechanical properties of heterogeneous HBSs and develop an efficient method to determine their strength parameters. In this study, heterogeneous hydrate-bearing clayey sand specimens were prepared by controlling the water distribution in host specimens. The hydrate-bearing layers in the prepared specimens have similar hydrate saturation, different thicknesses and angles. Multistage triaxial experiments were conducted to investigate the effects of hydrate-bearing layer thickness (HBLT) and angle (HBLA) on the stress-strain behavior and strength parameters of the specimens under differing effective confining stresses. The results show that the specimens produce bulging deformation and strain-hardening failure mode. Within the study scope, the strain-hardening degree of the stress-strain curve decreases with increasing HBLT and decreasing HBLA. The failure strength of the specimen initially increases and then decreases with increasing HBLT, peaking at 6 cm HBLT; however, it is positively correlated with HBLA. As HBLT increases, the cohesion increases exponentially, while the internal friction angle initially increases and then decreases. HBLA contributes to a linear increase in both cohesion and internal friction angle. Empirical equations for the cohesion and internal friction angle of heterogeneous specimens were formulated by replacing the heterogeneity with homogeneous elements, and their validity was verified. Under high effective confining stress, in the initial deformation stage, the shear stress applied to specimens with HBLT larger than 6 cm gradually increases with the increasing HBLT, while the thickness of the hydrate-free layer, which is susceptible to deformation response under external load, decreases. Consequently, the number of broken and damaged hydrates in these specimens increases. In the following shear process, failed hydrates prevent the pores from being compressed and serve as lubrication, leading to a reduction in the strain-hardening degree of the stress-strain curve and a reduction in failure strength. HBLA changes the contact area between hydrate-bearing and hydrate-free layers, as well as the axial distribution of hydrates, thus influencing the mechanical behavior of the specimen.