Geomechanical Response of Known Permafrost Hydrate Deposits to Depressurization and Thermal Loading

Authors: Jonny Rutqvist (speaker), George J. Moridis, and Timothy Collett

Venue: 6th International Conference on Gas Hydrates 2008, Vancouver, British Columbia, July 9-12, 2008. (http://www.icgh.org [external site]).

Abstract: The geomechanical response of Hydrate-Bearing Sediments (HBS) is a serious concern that needs to be addressed before the installation of facilities for hydrate deposits can proceed, and if gas production from hydrate deposits is to become reality. HBS are often unconsolidated, and are characterized by low shear strength. Heat from external sources, that cross the formation or depressurization-based production, can induce dissociation of hydrates (a strong cementing agent), and degradation of the structural stability of the HBS. Changes in pressure and temperature, phase changes, and the evolution of an expanding (and structurally weak) gas zone can significantly alter the distribution of loads in the sediments. The corresponding changes in the local stress and strain fields can result in substantial changes in the hydrologic, thermal and geomechanical properties of the system, displacement, and potentially failure.

This 3D numerical study of wellbore stability investigated the coupled hydraulic, thermodynamic and geomechanical behavior of permafrost HBS, that are either under production or are affected by warm ascending fluids, that are produced from deeper conventional reservoirs. The HBS properties and conditions in the simulations were from two known deposits at the Mallik site (Makenzie Delta, Northwest Territories, Canada), and at the Mount Elbert site (North Slope, Alaska, USA). The simulations were based on an elastoplastic Mohr-Coloumb model. Because of the dearth of information on the geomechanical properties of HBS, appropriate estimates were obtained from literature data based on the texture of sediment cores and/or related laboratory studies. The results of the study determined the range of properties and conditions under which geomechanical failure is likely at the two sites, provided estimates of the expected subsidence, and helped establish the envelope of stability for the safe operation of wells in these permafrost hydrate deposits.

Related NETL Project:
This presentation is related to the NETL project G308-01, “Numerical Studies for the Characterization of Recoverable Resources from Methane Hydrate Deposits.” The objective of this project is to develop a reservoir model that simulates the behavior of hydrate-bearing geologic systems and evaluates appropriate hydrate production strategies for both permafrost and marine environments, including thermal stimulation, depressurization and dissociation induced and/or enhanced by inhibitors (such as brines and alcohols). This research will enhance natural gas hydrate research and development activities by bringing new numerical simulation capabilities and laboratory measurements to bear on the difficult problems of characterization and gas recovery of methane hydrate deposits.

Project Contacts
NETL – Richard Baker (richard.baker@netl.doe.gov or 304-285-2714)
LBNL – George Moridis (GJMoridis@lbl.gov or 510 486-6709)