A Modeling Study of Geomechanical Performance of Sloping Oceanic Hydrate Deposits Subjected Production Activities
Authors: Rutqvist Jonny , Moridis George J. , Chiaramonte Laura 
Published: July 2010

Abstract
Oceanic Hydrate-Bearing Sediments (HBS) subjected to production activities can induced strongly coupled fluid flow and geomechanical changes that may have adverse consequences on wellbore integrity, the HBS, and the bounding formations. For a sloping hydrate bearing system, the geomechanical changes could potentially be catastrophic if triggering large slides of the seafloor. In this study we investigate two cases of coupled hydraulic, thermodynamic and geomechanical behavior of sloping oceanic HBS:hydrate heating as warm fluids from deeper conventional reservoirs ascend to the ocean floor through uninsulated pipes intersecting an HBS adjacent to the seafloor,system response during gas production from a hydrate deposit located 400 m below the seafloor. In the first case we consider a situation when a hydraulically confined hydrate layer near the ocean floor is dissociated by heating from the well with associated gas release and forced pressure increase, which could undermine the stability of the slope. In the second case, depressurization leads to hydrate dissociation over a large lateral distance that may provide a weakening potential slip surface along the sloping hydrate system. The results of our analysis indicate that although these production activities induces strongly coupled geomechanical responses that may adversely impact the production itself, they are unlikely to trigger large scale slides of the seafloor.

Introduction
Oceanic Hydrate-Bearing Sediments (HBS) subjected to production activities can result in changes in pressure and temperature with associated hydrate dissociation that may have adverse consequences on the integrity and stability of the wellbore assembly, the HBS, and adjacent formations. For a sloping hydrate bearing system, the geomechanical changes could be catastrophic with large scale slides of the seafloor. Marine sediments adjacent to the seafloor as well as hydrate deposits that are suitable target for production often involves unconsolidated sediments of relatively high porosity that are usually characterized by limited shear strength. The dissociation of the solid hydrates (a strong cementing agent) during gas production can degrade the structural strength of the HBS and thereby undermine the stability of the seafloor.
In this study we investigate two cases of coupled hydraulic, thermodynamic and geomechanical behavior of sloping oceanic HBS:hydrate heating as warm fluids from deeper conventional reservoirs ascend to the ocean floor through uninsulated pipes intersecting an HBS adjacent to the seafloor,system response during gas production from a hydrate deposit located 400 m below the seafloor. Similar cases were studied by Rutqvist and Moridis1, but in those cases a perfectly flat system with no potential for slide. That simulation study showed that stability of HBS in the vicinity of warm pipes may be significantly affected and that the formation around the wellbore is substantially weakened by the he heated and associated simultaneous pressure increase and hydrate dissociation. In a sloping system, such weakening could potentially trigger slide. For the case of depressurization-based gas production from oceanic hydrate deposit, the main concern would be the vertical compaction as well as increased shear stresses within the reservoir and the bounding formations. Moreover, the reservoir stresses changes quite rapidly along with the depressurization extending several hundred meters from the production well within weeks of production. Along with the production, the hydrate dissociates starting at the injection well and propagating preferentially along the upper and lower bounding shales. In a sloping system, such weakening of the sediment over a large lateral distance may provide a weak sediment layer surface prone to shear slip.