DE-FC26-05NT42664 / ESD05-036

Goal
The goal of this project is to develop the necessary knowledge base and quantitative predictive capability for modeling the geomechanical performance of hydrate-bearing sediments (HBS) 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.

Performers
Texas Engineering Experiment Station - College Station, TX

University of California Berkeley - Berkeley, CA

Lawrence Berkeley National Laboratory (LBNL) - Berkeley, CA

Schlumberger - Houston, TX

Background
Gas hydrates exist in many configurations below the sea floor, including massive (thick solid zones), continuous layers, nodules, and as widely disseminated interstitial material. Each of these hydrate accumulations may affect the seafloor stability differently. The hydrates in any of these cases may be a part of the solid skeleton that supports overlying sediments, which in turn support the platforms and pipelines needed for producing conventional oil and gas resources, as well as natural gas from hydrates (when this becomes economically and technically possible).

Accordingly, the potential instability of HBS is a subject of critical importance, and past researchers have described the conditions under which hydrate dissociation in HBS produces an enhanced fluidized layer at the base of the gas-hydrate zone. Submarine slope failure can follow, giving rise to debris flows, slumps, slides, and collapse depressions such as described by Dillon, et al. (1998). Failure would be accompanied by the release of methane gas, but a portion of the methane is likely to be oxidized unless the gas release is catastrophic.

Diagram showing the effects of gas hydrate dissociation on oceanic hillslope failures and gas release. Adapted from McIver (1982).

As a result of this potential for submarine sediment dislocation, the placement of wells and seafloor-grounded platforms associated with oil and gas production is strongly influenced by the presence of gas hydrate on the sea floor or within the sediment lithology. The primary concern is that warm fluids rising in a wellbore from deeper reservoirs may cause gas hydrate in the neighborhood of a well or pipeline to dissociate, reducing the stability of the supporting sediments and placing significant investments at risk. Such concerns would only increase if the hydrate accumulations are themselves the target of development operations. Locating platforms at sites dictated by the need to avoid hydrates?rather than optimize production operations, as is the current practice?increases the cost of production and impedes the commercial development of such deposits.

Currently, there is a lack of understanding of the mechanical and thermal properties of sediments containing gas hydrates, especially in marine deposits. Improving our ability to model the behavior of such sediments will improve the industry?s ability to make decisions related to the siting of production platforms, wells, and pipelines required to develop commercial hydrate deposits.

Potential Impact
This effort has the potential to have a significant impact on and provide substantial benefits to the offshore energy recovery industry, both in terms of current conventional oil and gas production operations and in the case of future production from hydrates. By establishing the principles of the geomechanical behavior of HBS and developing numerical codes to evaluate this behavior under a variety of conditions, the knowledge gained from this study will be instrumental in predicting and analyzing the stability of hydrate-bearing media in the ocean subsurface. This capability will provide valuable input for the selection of appropriate sites for offshore platform installation, as well as for the design and operation of production platforms.

Accomplishments
Under Phase 1 Researchers:

• Completed and submitted an initial Research Management Plan outlining planned project activity, spending, and schedule.
• Completed and submitted a Technology Status Assessment.
• Completed a literature survey and generated a summary report on characteristics of sediments containing gas hydrates in the ocean.
• Completed development of a conceptual pore-scale model based on available data and reports. After testing several approaches, one has been selected based on the most comprehensive contact mechanics.
• Initiated preliminary concept testing for a pore scale model on simple configurations with verification of the results against known measurements and observations Current work in this area involves incorporating tangential forces into the pore scale model.
• Completed development of FLAC 3D routines required for TOUGH+/Hydrate (T+H)/FLAC 3D integration.
• Completed development of the T+H/FLAC 3D interaction interface.
• Completed integration and final testing of the coupled geomechanical numerical model T+H/FLAC 3D.
• Initiated demonstration that Petrel can be used to develop an earth model for providing data to the T+H/FLAC3D model and demonstrated that surfaces can be transferred from Petrel to FLAC 3D.
• Completed Phase 1 effort and reporting.

Under Phase 2 Researchers:

• Completed design and manufacturing of a specialized triaxial cell that is transparent to x-rays and initiated geophysical analyses and stress/strain geomechanical studies of hydrate bearing sediment (This work is to be continued under LBNL field work proposal ESD05-048)
• Completed a Topical Report outlining the methodology proposed for creation of synthetic hydrate-bearing sediment samples in progressively finer-grained media. The samples will be used in the performance of Phase 2 geomechanical laboratory studies.
• Trained two graduate students (TEES and Colorado School of Mines) in the use of the TOUGH+HYDRATE numerical simulator as well as FLAC3D for the coupled analysis of flow, thermal and geomechanical processes in hydrate-bearing media.
• Conducted multiple numerical simulations to model behavior of hydrate and hydrate bearing sediment (both thermodynamic and geomechanical) from various classes of hydrate reservoir and for traditional oil and gas production through hydrate bearing sediment.
• Conducted efforts to develop grid creation methodologies necessary for linking Petrel with FLAC 3D as well as the methodology for extracting material properties from Petrel for use in assigning properties to FLAC 3D grid elements.
• Extended quasi-static grain-scale model of HBS to incorporate tangential contact forces at grain-to-grain contacts and the enhancement of pore scale modeling run efficiency through implementation of customized algorithms.
• Conducted fundamental studies of pore-scale geomechanical behavior?verifying the model developed in Phase 1 by comparing the numerical simulation results against laboratory data.

An abrupt local change in force chains and configuration, affecting the overall response of a small pack (306 grains). The plot shows 4 consecutive configurations (marked 1- 4), where the macroscopic strains applied in each step are similar. In each plot the maximum (top 10%) force vectors are plotted with the line width proportional to the contact force magnitude. Also shown is a cluster of grains. Note the abrupt change

Current Status
All work to be performed under this specific project (through Phase 2) is complete and the results of that work are provided in the Final Report accessible from the "Additional Information" section below.

The achievements of the project are outlined in the Accomplishments section above. This project has been ended at the completion of project Phase 2 and the project will not progress into the originally planned Phase 3. Laboratory and modeling work currently being conducted under field work proposal ESD05-036, as a part of this project, will be continued but will be carried out under field work proposals ESD05-048 and G308 with LBNL.

Coupling of TOUGH+HYDRATE and FLAC3D for the analysis of geomechanical behavior of hydrate-bearing sediments

Project Start: October 1, 2005
Project End: April 30, 2008

DOE Contribution: NT42664, $452,426; ESD05-036, $240,000
Performer Contribution: NT42664, $180,000

Contact Information
NETL ? Rick Baker (richard.baker@netl.doe.gov or 304-285-4714)

NT42664
TEES / TAMU ? Steve Holditch (holditch@tamu.edu or 979-845-2255)

ESD05-036
Lawrence Berkeley National Laboratory - George Moridis (gjmoridis@lbl.gov or 510-486-4746)

Additional Information:
In addition to the information provided here, a full listing of project related publications and presentations as well as a listing of funded students can be found in the Methane Hydrate Program Bibliography [PDF].

Final Project Report [PDF-8.76MB]

Semi-Annual Report  [PDF-2.67MB] - April - September, 2007

Topical Report [PDF-94KB] - Approach to Forming Hydrate Bearing Samples in Fine Grained Material

Offshore Technologies Conference paper [PDF-2.42MB] - May, 2007 - Numerical Studies of Geomechanical Stability of Hydrate-Bearing Sediments

Semi-Annual Report  [PDF-145KB] - October, 2006 - March, 2007

Phase 1 Topical Report  [PDF-5.72MB] - December, 2006

Semi-Annual Report  [PDF-854KB] - April - September, 2006

Phase 1 Status Presentation [PDF-1.84MB] Lawrence Berkeley National Laboratory - January 19, 2007

Phase 1 Status Presentation [PDF-2.88MB] Schlumberger - January 19, 2007

Phase 1 Status Presentation [PDF-2.18MB] Texas A&M University - January 19, 2007

Phase 1 Status Presentation [PDF-2.28MB] University of California Berkeley - January 19, 2007

Semi-Annual Report for period October 1,2005 - March 31,2006 [PDF-375KB]

Semi-Annual Report for period October 1,2005 - March 31,2006 - Appendix [PDF-200KB]

Technology Status Assessment [PDF-141KB]

Pertinent Publications:
Government Report
Holditch, S., T. Patzek, G. Moridis, and R. Plumb, 2005, Geomechanical Performance of Hydrate-Bearing Sediments in Offshore Environments, U.S. DOE-NETL Semi-Annual Report, October 1, 2005 through March 31, 2006, DE-FC26-05NT42664 CFDA Number: 81.089 (Fossil Energy Research and Development), July, available online [PDF-375KB] and Appendix [PDF-200KB] .

Presentations
Holtzman, R., D. Silin, T. Patzek, 2006, The Strength of Hydrate-Bearing Sediments: A Grain-Scale Approach, AGU Fall Meeting, San Francisco, CA, December 15.