Coupled Hydrological, Thermal and Geomechanical Analysis of Wellbore Stability in Hydrate-Bearing Sediments (OTC 19672)

Authors: Jonny Rutqvist (speaker), George J. Moridis, and Tarun Grover

Venue: 2008 Offshore Technology Conference, Houston, Texas, May 5-8, 2008 ( http://www.spe.org and http://www.smenet.org [external sites] )

Abstract: This study investigated coupled multiphase flow, themal, thermodynamic and geomechanical behavior of oceanic Hydrate Bearing Sediments (HBS), during depressurization-induced gas production in general, and potential wellbore in-stability and casing deformation in particular. The project investigated the geomechanical changes and wellbore stability for two alternative cases of production using a horizontal well in a Class 3 deposit and a vertical well in a Class 2 deposit. The research compared the geomechanical responses and the potential adverse geomechanical effects for the two different cases. Analysis shows that geomechanical responses during depressurization-induced gas production from oceanic hydrate deposits is driven by the reservoir-wide pressure decline (Delta P), which in turn is controlled by the induced pressure decline near the wellbore. Because any change quickly propagates within the entire reservoir, the reservoir wide geomechanical response can occur within a few days of production induced pressure decline.

The study shows that there is a major difference in the geomechanical performance around horizontal and vertical wells. In the case of production from horizontal wells, the anisotropic stress induced by the general reservoir depressurization can cause shear failure near the wellbore adjacent to the perforation. For production from a vertical well on the other hand, the formation will be unloaded uniformly in a plane normal to the axis of the wellbore. The load on the wellbore casing will decrease, and failure of the formation around the perforation will be prevented. In the case of a horizontal well, the main concern is increased compression (load) against the upper part of the wellbore casing caused by the compacting reservoir. This compressive load first caused local shear failure (yielding) in the formation leading to loss of bonding between grains, which may lead to production of solid sediment particles and formation of cavities around the perforation. Analysis shows that for reasonable strength proper-ties of an oceanic HBS, there is a very high potential for such localized shear failure. In the case of a vertical well, the main concern is the vertical settlement of the formation, which may be substantial, especially in the vicinity of the well where pressure is the lowest. Finally, the analysis shows that the failure of the formation during depressurization-induced gas production is likely to occur at relatively high effective stress. Investigation of the strength behavior of HBS should be conducted at such appropriate confining stress range, including the possibility of pore-collapse.

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 304-285-4714)
LBNL – George Moridia (GJMoridis@lbl.gov or 510-486-4414)