Numerical Simulation of Failure Properties of Interbedded Hydrate-Bearing Sediment and its Implications on Field Exploitation

Abstract

Marine natural gas hydrate is widely accepted as a promising substitute energy resource, whereas geotechnical responses of hydrate-bearing sediments (HBS) are crucial for the safe development of natural gas hydrate. The hydrate spatial distribution in sediments is multitype and present different coalescence degrees, its distribution pattern significantly influences the failure behavior of HBS, but the mechanics behind this influence is unclear.  Numerical simulations are of great importance in revealing such mechanics. In this paper, a series of numerical simulations, using a modified Mohr–Coulomb (MC) model, were performed to analyze the deformation mechanisms of HBS when the hydrate was laminarly distributed in its host sediment. Considering the vertical heterogeneity of the hydrate distribution, the vertical-laminar hydrate distribution samples were simplified as interbedded samples. The interbedded HBS specimens consisted of three layers: the middle layer with relatively lower hydrate saturation; and the upper and lower layers with relatively high hydrate saturation. Interbedded samples with different hydrate saturations, depressurization ratios, and thickness ratios were designed to analyze their mechanical behaviors. The results indicated that in both the hydrate saturation and thickness ratio of the interbedded samples, the plastic yield area first occurred in the lower hydrate saturation layer; the failure mechanism was mainly determined by the strength of the lower hydrate saturation layer. In addition, the shear failure behavior of the HBS was controlled by the hydrate saturation difference between the middle and upper layers. With a smaller hydrate saturation difference between the middle and upper layers, the samples were more likely to experience shear failure. In the thickness ratio samples, the HBS changed from brittle to ductile failure with an increase in the thickness of the lower hydrate saturation layer. Under different depressurization amplitudes, the larger compression and shear yield zones were mainly confined in the upper layer. This study provides a theoretical framework for controlling the strength-weakening mechanism during natural gas development , which can be used to guide the selection of depressurization schemes, monitor decomposition zones, and characterize reservoirs.

Keywords: natural gas hydrate, interbedded hydrate-bearing sediment, hydrate saturation, strength-weakening mechanism, depressurization amplitude