The goal of this research project was 1) to develop a methodology for estimating gas hydrate concentrations in deep-water, near-seafloor strata, 2) to estimate geomechanical properties of near-seafloor strata using seismic velocity data and 3) to show how mechanical properties vary with changes in hydrate concentration, mineralogy, and porosity

The University of Texas at Austin Bureau of Economic Geology, Austin, TX 78713-8924

The research combined four-component, ocean-bottom-cable (4-C OBC) seismic data, high-resolution chirp-sonar data, in-situ seafloor observations, and new rock physics concepts to predict, detect, and map the occurrence of deep-water gas hydrates and to estimate the concentration of hydrate in deep-water environments. The research methodology that was developed will allow key geomechanical properties of near-seafloor strata to be estimated from P- and S-wave velocity data. The results provided information on how the mechanical properties of these sediments vary with hydrate concentration, mineralogy, effective pressure and porosity. This information is critical for understanding how chemical processes involved in hydrate formation and dissolution affect seafloor stability and drilling operations.

The results of this research will advance our ability to assess deep-water gas hydrates as a future energy resource and will provide valuable detail about the stratigraphic architecture and sedimentary fabric of deep-water gas hydrate systems. In addition, the geomechanical estimates may help us understand the relationship between seafloor stability and the free-gas-to-hydrate transition in marine hydrate systems.

Suspected hydrate occurrences in the deep-water area of the Green Canyon region on the northern shelf of the Gulf of Mexico (GOM) were confirmed using seismic data and resistivity log data; two hydrate-bearing study sites were selected for further investigation; rock physics models were developed and tested for hydrate-bearing sediments in deep-water, near-seafloor strata. Joint inversions of resistivity-log data and seismic-based VP velocities were carried out at calibration wells at each study site and yielded estimates of hydrate concentration indicating hydrate occupies a significant portion of the available pore space. In addition, P-P and P-SV images of near-seafloor geology were constructed along 200 km of OBC seismic profiles at the study sites.

During the first phase of this research, the focus of the project was to determine evidence that hydrate concentrations occur along the deep-water area of the Green Canyon region on the northern shelf of the GOM. Preliminary work showed that the P-SV mode of 4-C OBC seismic data allows near-seafloor geology to be analyzed at resolutions of approximately meter scale. This resolution has heretofore been unheard of using seismic data having frequency spectra of 10-100 Hz. More important, combinations of P-P and P-SV seismic data can be used to map hydrate-sensitive Vp/Vs attributes and key elastic moduli across targets of interest.

Resistivity log data was also analyzed across the study area to confirm the existence of hydrates. The performer found that the classical Archie saturation equation used to interpret hydrocarbon concentration in consolidated media can also be used to predict hydrate concentration in high-porosity, unconsolidated sediments if appropriate constants are used to adjust the equation response to the resistivity of the medium that hosts deep-water hydrates. These modifications, which account for clay-rich fractions of the media, should make the equation more appropriate and accurate for estimating gas hydrate concentrations.

Based on these analyses, evidence of hydrate occurrence was confirmed and two study sites in the Green Canyon area were selected for further investigation. In addition to evidence of hydrate occurrence, these sites were selected due to an abundance of available data (e.g., well log and seafloor borings) necessary to calibrate seismic attributes with sub-seafloor sediment and hydrate properties estimated from well data.

During Phase 2, the project team compiled a database consisting of 4-C OBC seismic data, chirp-sonar profiles, and in situ seafloor core and sediment sample analyses in the two Green Canyon study areas and developed specialized software for maintaining and manipulating the data.

During Phase 3, the research team focused on (1) processing the 4C OBC seismic data that traverse the two study sites, and (2) expanding rock physics modeling to calculate Vp and Vs velocities in media that have variable percentages of hydrate, quartz, and clay. Resistivity logs across the local hydrate stability zone of the two study sites were re-examined in order to ensure accurate estimates of hydrate concentration for calibration to hydrate concentrations from the seismic estimates. In addition, the research team evaluated conceptual models of hydrate-bearing systems,selected the rock physics model best-suited to hydrates occurring in the study area, and converted the best-fit conceptual model to a working computer code. All parameters needed to invert resistivity data and velocity data to estimates of hydrate concentration have been expressed as probability distribution functions (PDFs), which allowed hydrate estimations to be produced as PDFs. The mean value of the PDF obtained by this inversion indicates the magnitude of hydrate concentration, and the standard deviation of the PDF is a measure of the accuracy of the hydrate prediction.

During phase 4, the project focused on rock physics principles/models previously identified to describe relationships between P and S velocity attributes in deep-water sediments and concentrations of gas hydrates in those sediments. Hydrate occurrences investigated included (1) hydrates disseminated in pore spaces and acting as load-bearing support, (2) pore-filling hydrate, (3) hydrate-rich sediment layers interbedded with hydrate-free sediment layers, and (4) thin layers of pure hydrate embedded in sediment laminae and fractures.

Using 4-C OBC seismic data, the project team built a library of prestack and postack P-P and P-SV seismic attributes. Using results from previous tasks, comparison of theoretical predictions from the rock-physics models with actual seismic data were completed to determine which distribution scenario for the gas hydrates at each study site is most likely. Further, rock-physics-based models of P-P and P-SV attributes were utilized to construct estimates of spatial distributions of gas-hydrate concentrations in near-seafloor strata across selected study sites.

(July 2009) 
The research team has completed all planned activity under the project. The final project report is available below under "Additional Information"..

Phase 1: $457,164
Phase 2: $367,174
Total Funding:
DOE Contribution: $824,338

Phase 1: $106,312
Phase 2: $109,463
Total Funding:
Performer Contribution: $215,775

NETL – John Terneus (john.terneus@netl.doe.gov or 304-285-4254)
University of Texas - Dr. Bob Hardage, UT Austin (bob.hardage@beg.utexas.edu or 512-471-0300)

Awards:
President's Certificate for Excellence, Energy Minerals Division, American Association of Petroleum Geologists, for "Seismic Estimation of Gas Hydrate Concentrations in Deepwater Environments: Assumptions and Limitations," 2006

SEG Best Paper Award for "High-resolution multicomponent seismic imaging of deepwater gas-hydrate systems," published in May 2006. Award to be presented to Dr. Hardage and co-authors at SEG's 2007 Annual Meeting in San Antonio, Texas, Sept 23-28th, 2007.

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-23MB] - October, 2009

Technical Status Assessment [PDF-28KB]

Phase 1 Technical Report [PDF-798KB]

Phase 1 Technical Report (Research Database) [PDF-2.17MB]

Phase 2 Technical Report [PDF-5.04MB]

Project Continuation Report [PDF-1.67MB]

September 2007 Project Review [PDF-4.58MB]

2008 ICGH Paper - Assessing Fluid-Gas Expulsion Geology and gas Hydrate Deposits Across the Gulf of Mexico with Multicomponent and Multifrequency Seismic Data [PDF]

SEG 2008 Abstract [PDF-202KB]

Peer-Reviewed Publications
Sava, D., and B. Hardage, in review, Rock-physics models for gas-hydrate systems associated with unconsolidated marine sediments, in Collett, T., A. Johnson, C. Knapp and R. Boswell, eds., Natural Gas Hydrates: Energy Resource and Associated Geologic Hazards, The American Association of Petroleum Geologists Hedberg Special Publication.

Hardage, B.A., Roberts, H. H., Murray, P. E., Remington, R., Sava, D. C., Shedd, W., and Hunt, J. Jr.,Multicomponent seismic technology assessment of fluid-gas expulsion geology, Gulf of Mexico, , in Collett, T., A. Johnson, C. Knapp and R. Boswell, eds., Natural Gas Hydrates: Energy Resource and Associated Geologic Hazards, The American Association of Petroleum Geologists Hedberg Special Publication.

Government Reports
Hardage, B., M. Backus, M. DeAngelo, S. Fomel, R. Graebner, P. Murray, and L. Wood, 2002, Characterizing Marine Gas-Hydrate Reservoirs and Determining Mechanical Properties of Marine Gas-Hydrate Strata with 4-Component Ocean-Bottom-Cable Seismic Data, Final Report, DOE Contract No. DE-FC26-00NT41024.

Other Publications
Backus, M., P. Murray, B. Hardage, and R. Graebner, 2006, High-resolution multi-component seismic imaging of deepwater gas hydrate systems, The Leading Edge, Volume 25, n. 5, p. 578-596.

DeAngelo, M., M. Backus, B. Hardage, P. Murray, and S. Knapp, 2003, Depth registration of P-wave and C-wave seismic data for shallow marine sediment characterization, Gulf of Mexico, The Leading Edge, Volume 22, n. 2, p. 96-105.

Hardage, B., and P. Murray, 2006, Detailed imaging of deepwater hydrate geology with horizontal arrays of seafloor sensors, Proceedings of the Offshore Technology Conference, paper 17929.

Hardage, B., and P. Murray, 2006, High-resolution P-P imaging of deepwater near-seafloor geology, The American Association of Petroleum Geologists Explorer, Geophysical Corner, Volume 27, n. 7, p. 30.

Hardage, B., and P. Murray, 2006, P-SV data most impressive image, The American Association of Petroleum Geologists Explorer, Geophysical Corner, Volume 27, n. 8, p. 30.

Hardage, B., R. Remington, and H. Roberts, 2006, Gas hydrate--a source of shallow water flow?, The Leading Edge, Volume 25, n. 5, p. 634–636.

Hardage, B., and H. Roberts, 2006, Gas hydrate in the Gulf of Mexico: what and where is the seismic target, The Leading Edge, Volume 25, n. 5, p. 566-571.

Hardage, B., and H. Roberts, 2006, Evaluation of deepwater gas-hydrate systems, The Leading Edge, Volume 25, n. 5, p. 572-577.

McGee, T., and B. Hardage, 2006, Hydrate system to be monitored, The American Association of Petroleum Geologists Explorer, Geophysical Corner, Volume 27, n. 5, p. 24.

Roberts, H., B. Hardage, W. Shedd, and J. Hunt Jr., 2006, Seafloor reflectivity–an important seismic property for interpreting fluid/gas expulsion geology and the presence of gas hydrate, The Leading Edge, Volume 25, n. 5, p. 620–628.

Sava, D., and B. Hardage, 2006, Rock physics characterization of hydrate-bearing deepwater sediments, The Leading Edge, Volume 25, n. 5, p. 616-619.

Hardage, B. A., 2007, Gas hydrate and LNG tankers: AAPG Explorer, Geophysical Corner, v. 28, no. 5, p. 36.

Presentations
Backus, M., P. Murray, B. Hardage, and R. Graebner, 2005, Enhanced PS-wave images of deep-water, near-seafloor geology from 2-D 4-C OBC data in the Gulf of Mexico, 75th Annual International Meeting of the SEG, Houston, TX, November 6-11.

Hardage, B., and D. Sava, 2006, Seismic estimation of Gas Hydrate Concentrations in Deepwater Environments: Assumptions and Limitations, Houston, TX, The American Association of Petroleum Geologists Annual Meeting, April 9-12. (Winner of President’s Certificate for Excellence)

Hardage, B., 2006, Detailed imaging of deepwater hydrate geology with horizontal arrays of seafloor sensors, Houston, TX, Offshore Technology Conference, May 3.

Hardage, B., 2005, Assessing deep water gas hydrate systems and seafloor stability, invited lecture, New Orleans, LA, AGU, NABS, SEG, SPD/AAS Joint Assembly, May 23–27.

Hardage, B., and P. Murray, 2006, Detailed imaging of deepwater hydrate geology with horizontal arrays of seafloor sensors, paper 17929, 2006 Offshore Technology Conference, Houston, TX, May 1-4.

Murray, P., M. DeAngelo, B. Hardage, M. Backus, R. Graebner, and S. Fomel, 2005, Interpreting multicomponent seismic data in the Gulf of Mexico for shallow sedimentary properties: methodology and case history, Houston, TX, 2005 Offshore Technology Conference, May 2-5.

Sava, D., and B. Hardage, 2006, Rock physics models of gas hydrates from deep water, unconsolidated sediments, 76th Annual Meeting of the Society of Exploration Geophysicists, New Orleans, Louisiana, October 1-6.