Numerical Studies on the Geomechanical Stability of Hydrate-Bearing Sediments
Authors: Rutqvist Jonny , Moridis George J. 
Published: October 2008

Abstract
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, have 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 in three cases of coupled hydraulic, thermodynamic and geomechanical behavior of oceanic hydrate-bearing sediments. 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 hydrate-bearing sediments.
For the analysis of the geomechanical stability of HBS, we developed and used a numerical model that integrates a commercial geomechanical code and a simulator describing the coupled processes of fluid flow, heat transport and thermodynamic behavior in the HBS. Our simulation results indicate that the stability of HBS in the vicinity of warm pipes may be significantly affected, especially if the sediments are unconsolidated and more compressible. 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.

Introduction

Background.
Gas hydrates are solid crystalline compounds in which gas molecules (referred to as guests) are lodged within the lattices of ice crystals (called hosts). Under suitable conditions of low temperature and high pressure, a gas G will react with water to form hydrates according to G + NH H2O = G+NH H2O, where NH is the hydration number. Of particular interest are hydrates formed by hydrocarbon gases when G is an alkane. Natural hydrates in geological systems also include CO2, H2S and N2 as guests. Vast amounts of hydrocarbons are trapped in hydrate deposits1. Such deposits occur in two distinctly different geologic settings where the necessary low temperatures and high pressures exist for their formation and stability: in the permafrost and in deep ocean sediments. The three main methods of hydrate dissociation are2:depressurization, in which the pressure P is lowered to a level lower than the hydration pressure Pe at the prevailing temperature T,thermal stimulation, in which T is raised above the hydration temperature Te at the prevailing P, andthe use of inhibitors (such as salts and alcohols), which causes a shift in the Pe-Te equilibrium through competition with the hydrate for guest and host molecules.
Dissociation of the solid hydrate phase results in the production of gas and water.