Numerical Studies of Geomechanical Stability of Hydrate-Bearing Sediments

Authors: George J. Moridis, Jonny Rutqvist, Lawrence Berkeley National Laboratory.

Venue: 2007 Offshore Technology Conference, Houston, TX, April 30–May 1, 2007 (http://www.otcnet.org/ [external site]).

Abstract: The thermal and mechanical loading of 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. These factors can also impede the development of hydrate deposits as gas resources. For the analysis of the geomechanical stability of HBS, project researchers developed and used a numerical model that integrates a commercial geomechanical code into a simulator describing the coupled processes of fluid flow, heat transport, and thermodynamic behavior in geologic media. The geomechanical code includes elastoplastic models for quasi-static yield and failure analysis and viscoplastic models for time-dependent (creep) analysis. The hydrate simulator can model the non-isothermal hydration reactions (equilibrium or kinetic), phase behavior, and flow of fluids and heat in HBS, and can handle any combination of hydrate dissociation mechanisms. The simulations can account for the interdependence of changes in the hydraulic, thermodynamic, and geomechanical properties of the HBS, in addition to swelling/shrinkage, displacement (subsidence), and possible geomechanical failure. Researchers investigated in three cases the coupled hydraulic, thermodynamic, and geomechanical behavior of oceanic HBS systems. 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 involves mechanical loading caused by the weight of structures placed on HBS at the ocean floor, and the third describes system response during gas production from a hydrate deposit. The results indicate that the stability of HBS in the vicinity of warm pipes may be significantly affected, especially near the ocean floor where the sediments are unconsolidated and more compressible. Conversely, the increased pressure caused by the weight of structures on the ocean floor increases the stability of hydrates, while gas production from oceanic deposits minimally affects the geomechanical stability of HBS under the conditions that are deemed desirable for production.

Related NETL Project: The goal of the related NETL project entitled “Geomechanical Performance of Hydrate-Bearing Sediments in Offshore Environments ” (ESD05-036 and DE-FC26-05NT42664) is to develop the necessary knowledge base and quantitative predictive capability for modeling the geomechanical performance of hydrate-bearing sediments in oceanic environments, in particular to determine the envelope of hydrate stability under conditions typical of those related to the construction and operation of offshore platforms.

NETL Project Contacts:
NETL – Rick Baker (richard.baker@netl.doe.gov or 304-285-4714)
LBNL – George J. Moridis (GJMoridis@lbl.gov or 510-486-4746)
Texas A&M – Steve Holditch (holditch@tamu.edu or 979-845-2255)