Impact of Interbedded Structure of Sand and Clay Layers on Geomechanical Responses of Hydrate-bearing Sediments During Depressurization

Abstract

This study investigates how the variation in sediment layer geometry of hydrate-bearing sediments (HBS) affects geomechanical behaviors of HBS under depressurization. Two reservoir models with different layering structures but with the same hydrate quantity were constructed and the reservoir responses were numerically investigated during gradual depressurization process. To simulate thermo-hydro-mechanically coupled multiphysics processes occurring in HBS, a series of governing equations were discretized based on a finite volume concept, and coded into an explicit finite difference numerical simulator. An explicitly coupled, time-marching algorithm was used to couple thermo-hydro-mechanical responses associated with depressurization-driven hydrate dissociation. We herein modelded a hydrate deposit in Ulleung Basin, Korea for the sediment properties and geological setting. The simulation results clearly demonstrate that the "densely" layered HBS structure, composed of thin and interbedded clay-sand layers, is more prone to geomechanical instability though it led to more gas production. It is attributed to various mechanisms, including (i) the rapid water drainage from neighboring thin clay layers, (ii) the unique hydrate dissociation pattern in interbedded HBS, and (iii) the transfer of shear stress from hydrate-bearing, "stiff" sandy layers into adjacent thin "soft" clay layers. The layer geometry substantially affects not only the gas production but also the geomechanical stability of a hydrate reservoir. High-resolution sediment profiling appears to play an important role in numerical HBS simulations to reliably predict the feasibility of safe exploitation from layered HBS systems.