The goal of this project is to evaluate the direct-current electrical resistivity (DCR) method for remotely detecting and characterizing the concentration of gas hydrates in the deep marine environment. This will be accomplished by adapting existing DCR instrumentation for use on the sea floor in the deep marine environment and testing the new instrumentation at Mississippi Canyon Block 118.

Baylor University, Waco, TX 76798

Collaborators
Advanced Geosciences Inc., Austin, TX 78726
Specialty Devices Inc., Wylie, TX 75098

Marine occurrences of methane hydrates are known to form in two distinct ways. By far the most common occurrence is associated with the vertical migration of biogenetic gas into the near-bottom hydrate stability zone. Hydrates that form in this way are normally, but not always, associated with bottom simulating seismic reflections (BSR). In these cases the BSR signature indicates that gas hydrates are present over large areas, but seismic information alone is not enough to determine where concentration levels may be high enough to warrant future production consideration. The second kind of marine hydrate deposits form by the vertical migration of thermal gas from deep source rocks and conventional gas reservoirs. Thermally-derived hydrates are normally associated with gas seeps that occur where deep-seated faults intersect the sea-floor. They are generally not laterally extensive, but because the gas seeps are sites of highly focused methane discharge, greater concentrations of hydrate are possible. As a result of this concentration, thermal hydrate deposits may be the first in the marine environment to be considered for production. However, because thermal hydrates are seldom associated with BSR signatures, neither their presence nor their concentration can be reliably determined by seismic methods alone.

There is a growing consensus that additional geophysical information in the form of sub-bottom electrical resistivity data will be needed to confirm the presence and determine the concentration of gas hydrate. While the presence of hydrate in the sediment pore spaces causes only minor changes in seismic velocities, the electrical properties of sediment are greatly influenced by the presence of either hydrate or free gas. Hence, the occurrence of a high resistivity anomaly in a subsurface region associated with a seismic velocity anomaly would indicate the presence of free gas. Anomalously high resistivity in a region with essentially normal seismic velocities is indicative of the presence of hydrate. The question that remains is “What kind of electrical method will be most applicable to future hydrate exploration needs?”

This project will attempt to further the development of marine electrical profiling by adapting DCR methods developed for land-based and shallow-water environmental studies to hydrate characterization in the deep-marine environment.

The proposed geophysical method is potentially simpler, less expensive, and more easily extended to 3-D and 4-D surveys than geophysical methods previously applied to the study of gas hydrate deposits. If the experiments are successful and the DCR method demonstrates the ability to detect and characterize gas hydrate distribution at the test site, then the method could become another very important tool for hydrate characterization. When used in conjunction with existing and new seismic methods, this could represent a novel “combined technique” methodology for more effectively locating and characterizing marine hydrate occurrences. Its use in reconnaissance surveys could be particularly important when exploring for thermal hydrate deposits not associated with BSR signatures. Its use for long-term monitoring would be particularly important in monitoring hydrate production, much as 4-D seismic data are currently being used to monitor petroleum production.

• The seafloor array system was tested in a nearby freshwater reservoir. The array was deployed to its full 500 m length on the shallow bottom and the same measurements taken during the land survey were repeated. The test yielded successful results allowing researchers to design a low-noise preamp that will help boost signal levels.
• The reconfigured, high-resolution sea floor array system was successfully tested on land, side by side against a land resistivity system with the same recording parameters. The measurements from the two systems agreed to within one standard deviation of the measurement variability (about 5%).
• The new electrode array, designed to increase the resolution of the near-bottom seafloor, was delivered by the manufacturer (Underwater Systems) in November 2011.
• Electrode configurations for the new high-resolution resistivity array have been chosen on the basis of finite element forward modeling, simulating conditions at the MC118 field site.
• The project team used data from a recent multi-beam bathymetry survey of the site in final inversions of the 2-D DCR profiles to account for variations in seafloor topography. The final tasks (within Phase 1), which involved analysis of the DCR data acquired during the 2-D reconnaissance survey, and a topical report were completed in August 2009.
• Analysis of the 2-D reconnaissance data was completed.
• Initial processing of the 2-D DCR data was completed in July 2009. The results indicate that, for the most part, the methane vent area in MC 118 is underlain by non-hydrate bearing sediment to a depth of 120 m below the bottom. Typical resistivities ranged between 0.5 to 1.0 Ohm-m. However, localized high resistivity anomalies, ranging from 10 to 30 Ohm-m, with dimensions of 20 to 40 m in width and thickness, occurred near the sea-floor where profiles cross the trace of a previously mapped fault. Resistivities in this range suggest that the anomalies may be caused by massive hydrate blocks lodged within the fault zone.
• A replacement electrode array was fabricated during the remainder of 2008. The repaired DCR system was then used to conduct a 2-D reconnaissance survey of MC 118 in June 2009. The survey consisted of seven continuous resistivity profiles, totaling 26.4 km in length.
• An initial sea trial of the DCR system at Mississippi Canyon Block 118 (MC 118) was conducted in June 2008. The instrument was successfully launched and lowered to the sea-floor. On the sea-floor, the DC resistivity system was powered-up by command from the surface ship, but communication with the instrument was lost during the initialization procedure. Upon recovery, it was discovered that the electrode array had been attacked by sharks and cut in half. The test verified that the control/power interface between the ROV and DCR system works, but no resistivity data were collected.
• The control/power interface between the DCR and the Specialty Devices Inc. remotely operated vehicle (ROV) was completed and tested in May 2008.
• The sea-floor electrode array for the DCR system was completed in March 2008.
• Electronic components for the DCR system were completed in March 2007, and assembled into the pressure housing in June 2007.
• The pressure housing for the sea-floor DCR system electronic components was completed in February 2007.

(June 2013)
The project has been completed. The final report is available below under "Additional Information". 

Phase 1 - DOE Contribution: $138,199
Phase 2 - DOE Contribution: $115,650,
Planned Total Funding - DOE Contribution: $253,849

Phase 1 - Performer Contribution: $21,957
Phase 2 - Performer Contribution: $46,928

Planned Total Funding - Performer Contribution: $68,885

NETL – Skip Pratt (skip.pratt@netl.doe.gov or 304-285-4396)
Baylor University – John Dunbar (john_dunbar@baylor.edu or 254-710-2191)

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-4.29MB] - March, 2013

September 2007 Project Review [PDF-4.47MB]

Kick-off meeting presentation [PDF-3.28MB] - January 9, 2007

Technology Status Assessment [PDF-57KB] - December, 2006 - "Geophysical Exploration Methods for Gas Hydrates"