The goal of this project is to investigate the feasibility of using marine electromagnetic (EM) surveying as a tool for characterizing and quantifying the occurrence of hydrate in the seafloor section. This will be done by collecting field data and quantifying the resistivity of natural gas hydrate using laboratory measurements.
University of California San Diego - Scripps Institution of Oceanography, La Jolla, CA 92093
Lawrence Livermore National Laboratory (LLNL), Livermore, CA 94511-9900
United States Geological Survey, Menlo Park, CA 94025
Massachusetts Institute of Technology, Cambridge, MA 02139.
Gas hydrate holds promise as an energy resource, is a potential hazard to offshore drilling and development, can be a factor in seafloor slope stability, and remains an enormous global reservoir of methane (a potent greenhouse gas). For all of these reasons, understanding how to quantify the volume and distribution of hydrate in marine sediments is important. Although methane hydrates occur in vast quantities (the consensus estimate is 10,000 Gt), estimates of total global volume span four orders of magnitude, mainly because existing geophysical methods are not sensitive to bulk concentration of hydrate in the seafloor section. Notably, the use of the seismic bottom simulating reflector (BSR), although a sensitive indicator of the base of the hydrate stability field, is largely ineffective at predicting the amount of hydrate between the seafloor and the BSR.
Electromagnetic (EM) methods, on the other hand, are sensitive to the concentration and geometric distribution of hydrate, but the use of marine EM techniques to characterize hydrate is still in its infancy. Field trials carried out to date have been limited in scope and sophistication, and there is a lack of laboratory-derived relationships between petrophysical properties and EM measurements on which to relate conductivity to quantitative estimates of hydrate volume in the seafloor section.
This project carried out four EM surveys in the Gulf of Mexico (GOM) over areas where hydrate is known (or thought) to exist in the sedimentary section at water depths ranging from 800 to 3,000m. The project employed state-of-the-art EM imaging equipment and techniques that incorporated conventional offshore hydrocarbon exploration methods and newly developed methods designed specifically for the purpose of hydrate mapping. Additionally, the project will build new laboratory conductivity apparatus to make the first measurements of the electrical conductivity of natural methane hydrate as a function of temperature, pressure, pore water fraction, pore water chemistry, and sediment fraction, and will use these results to calibrate the resistivity data obtained from the field surveys in terms of hydrate concentrations. It is expected that the results of this research will include three case histories that can be used to advance the use of marine EM methods to map and quantify gas hydrate in the Gulf of Mexico, and will also provide a basis for the use of EM as a complementary tool for carrying out global studies focused on directly quantifying the distribution, concentration, and total volume of hydrate accumulations.
The impact of successfully demonstrating the ability of marine EM methods to quantify hydrate content in marine sediments will be significant, as no such method currently exists. Hydrate deposits are not stratigraphic and even extensive drilling provides only point measurements with no reliable methodology for interpolation or extrapolation to regions beyond the wells (although sophisticated analysis of 3-D seismic data has recently been used to indicate areas where hydrate occurs, if not the concentrations). However, if an area can be comprehensively surveyed using only a few days of ship time, or if continuous profiling can be carried out, this would represent the first steps toward the possibility of a direct assessment of the global inventory of hydrate and could contribute to the discovery of economically viable hydrate concentrations. The method to be developed also holds potential for monitoring the sequestration of carbon dioxide in seafloor sediments, where the formation of CO2 hydrate has been proposed as a natural capping mechanism to prevent escape of sequestered CO2.
A paper, highlighting the results of the project to date was presented at the International Conference on Gas Hydrates in Edinburgh, Scotland in July 2011.
A paper entitled “Electrical Properties of Polycrystalline Methane Hydrate” was published in Vol. 38 of Geophysical Research Letters. The paper examines the impedance and conductivity measurements of methane hydrate synthesized in the conductivity cell.
Geological modeling and hydrate volume estimation were completed for the MC118 survey site. Preliminary results indicate that there is a general concentration of hydrate between 40 % and 80% at the resistivity anomaly located at the SE crater of the MC118 site. Surveyed areas outside of the anomaly indicate very low volumes of predicted hydrate. Hydrate volume estimates at the remaining sites (WR 313, GC 955, and AC 818) will require 2D or perhaps 3D modeling and inversion due to the more complex bathymetry and subsurface structures existing at those locations.
Geological modeling and hydrate volume estimation was completed for the MC118 survey site.
Scripps has designed and constructed the conductivity cell and calibration tests using ice and a Teflon plug have been completed. The cell was then commissioned and shipped to Menlo Park to initiate experiments for growing hydrate in the cell. Hydrate has been synthesized during four separate experiments in the conductivity cell at Menlo Park. During the first experiment a thermocouple was installed to calibrate the growth of the hydrate in the cell. In the second experiment, electrical conductivity measurements were made during and after hydrate formation. During the third experiment, the temperature was increased incrementally to reduce the temperature gradient in the sample. The fourth experiment involved the synthesis of hydrate within a sediment mixture (50% ice and 50% OK-1 sand).
Apparent resistivity pseudosections of the OBEM data have been made for all survey areas. Preliminary interpretations with published results and the JIP project Leg 2 expedition have been made. The Vulcan data collected at Mississippi Canyon 118 have also been processed and researchers generated apparent resistivity depth sections based on frequency. The results from this survey give the most compelling argument that controlled source electromagnetic (CSEM) surveys are sensitive to the presence of hydrate in the Gulf of Mexico . The MC 118 area is rather conductive with a background resistivity of 0.5-1 Ohm-m and is generally featureless except at the SE crater. No constraints were placed on the intersecting tow lines and so the fact that three lines independently give a resistive body at the SE crater assures us that this is an attribute of the data due to geology. The EW line that crosses through the SE crater is overlain on chirp acoustic line 119 from Sleeper et al. (2006) in order to compare zones of acoustic blanking to electrical conductivity. The acoustic blanking, or wipeout zones, at MC 118 are attributed to authogenic carbonate as well as free gas and gas hydrate (Lapham et al. 2008). Carbonate rocks are present on the crater floors and have been noted in the SW crater (McGee et al. 2009 and 2008). The SE crater has a pavement of dead methanotrophic clams and there is no evidence for recent venting, which suggests that the conduit which once supplied methane to these clams became blocked, perhaps due to hydrate formation (McGee et al. 2009 and 2008). Researchers find that the SE crater resistor appears to have some depth extent and the acoustic blanking there is resistive. However, the region of acoustic blanking toward the SW crater has a background resistivity of 1 Ohm-m, attributed to the shallow carbonates present there (Macelloni, pers comm.). This is significant in that hydrate and carbonates, initially thought to be confounding electrical resistors, are in fact differentiable here. Only drilling at the SE crater will confirm that it is hydrate.
This project achieved a major technical objective in 2008. As part of a comprehensive study to develop marine electromagnetic methods for gas hydrate detection and mapping, the project has carried out an 18-day cruise on the R.V. Roger Revelle in the Gulf of Mexico from October 7 to 26, 2008. During this experiment, a total of 94 ocean bottom electromagnetic (OBEM) recorders were deployed at four survey areas (Alaminos Canyon block 818, Walker Ridge block 313, Green Canyon block 955, and Mississippi Canyon block 118) and the Scripps Undersea Electromagnetic Source Instrument (SUESI) was towed through the survey areas for a total of 103 hours. Data transmission was accomplished via a 200 A, 50 m dipole antenna at heights of 70–100 m above the seafloor. The field expedition vessel also towed a 3-axis electric field recorder behind the SUESI antenna at a constant offset of 300 m. Field data collection activities were highly successful with acquisition of high quality data on almost all deployments and with only two deployments failing to collect data.
The project ended on March 31, 2012. The PI gave a closeout presentation summarizing the results to NETL personnel. The final report is available below under "Additional Information".
Phase 1 – 12 months, $622,131
Phase 2 – 12 months, $145,738
Phase 3 – 12 months, $93,809
Total Funding - $861,679
Project Funding (FEW0160)
DOE Contribution: $82,000
Phase 1 – 12 months, $615,584
Phase 2 – 12 months, $70,908
Phase 3 – 12 months, $0
Total Funding - Performer Contribution: $686,492
NETL – Skip Pratt (email@example.com or 304-285-4396)
SCRIPPS – Steve Constable (firstname.lastname@example.org or 858-534-2409)
Lawrence Livermore National Laboratory (LLNL) – Jeffery L. Roberts (Roberts17@llnl.gov) or 925-422-7108
If you are unable to reach the above personnel, please contact the content manager.
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-29.7MB]
Summary of Phase II Research [PDF-7.66MB] - May 2011
Phase 1 Final Report [PDF-20.2MB] - October, 2009
Preliminary Cruise Report [PDF-14.7MB] - Report from October, 2008 GOM cruise
The Kick-Off Presentation for this project is available by request. It is a PDF file that is approximately 35MB.
Link to the Scripps GOM Hydrates Field Program Website [external site]