Numerical Studies on the Geomechanical Stability of Hydrate-Bearing Sediments

Abstract

Summary The thermal and mechanical loading of oceanic hydrate-bearing sediments (HBS) can result in hydrate dissociation and a significant pressure increase with potentially adverse consequences on the integrity and stability of the wellbore assembly, the HBS, and the bounding formations. The perception of HBS instability, coupled with insufficient knowledge of their geomechanical behavior and the absence of predictive capabilities, has resulted in a strategy of avoidance of HBS when locating offshore production platforms and can impede the development of hydrate deposits as gas resources. In this study, we investigate coupled (interacting) hydraulic, thermodynamic, and geomechanical behavior of oceanic HBS in three cases. The first involves hydrate heating as warm fluids from deeper conventional reservoirs ascend to the ocean floor through uninsulated pipes intersecting the HBS. The second case describes system response during gas production from a hydrate deposit, and the third involves mechanical loading caused by the weight of structures placed on the ocean floor overlying the HBS. For the analysis of the geomechanical stability of HBS, we developed and used a numerical simulator that integrates a commercial geomechanical simulator and a simulator describing the coupled processes of fluid flow, heat transport, and thermodynam-ic behavior in the HBS. Our simulation results indicate that the stability of HBS in the vicinity of warm pipes may be affected significantly. Gas production from oceanic deposits may also affect the geomechanical stability of HBS under the conditions that are deemed desirable for production. Conversely, the increased pressure caused by the weight of structures on the ocean floor increases the stability of underlying hydrates.