The overall objectives of this work are (i) advance understanding of hydrate electrical conductivity as a function of sediment type and fluid content; (ii) quantify the conductivity changes associated with hydrate dissociation induced by increasing temperature or decreasing pressure; and (iii) collect baseline data sets in the field to illustrate the capabilities of the Vulcan instrument system, calibrate the relationship between conductivity inversions and well logs, and provide quantitative constraints on hydrate volume in situ.
Phase 1 objectives: Understand the effect of grain size on methane hydrate conductivity. Assess the impact on methane hydrate conductivity of dissociation associated with (a) decompression and (b) increased temperature. Image the electrical conductivity structure of 2 or 3 prospects in the Gulf of Mexico (GoM) using the Vulcan marine Controlled Source ElectroMagnetic (CSEM) system.
Phase 2 objectives: Interpret the Vulcan inversions to obtain quantitative estimates of total hydrate volume.
Phase 3 objectives: Complete the integration of field interpretations, laboratory conductivity studies, and any available logging/coring results. Publicize results and facilitate commercial application of the technology.
The Regents of the University of California – San Diego (UCSD), Scripps Institute of Oceanography, La Jolla, CA 92093
United States Geological Survey (USGS), Menlo Park, CA 94025
Lawrence Livermore National Laboratory, (LLNL), Livermore, CA 94551
In order to clarify the processes by which gas hydrate deposits are formed, maintained, and evolve within geologic systems, it is important to develop tools other than drilling, seismology, and geochemistry to study hydrate systems, both in the field and in the laboratory. Much progress has been made in our understanding of hydrate systems using the existing tools, but adding electrical conductivity cannot fail to increase our understanding of gas hydrate systems. Combined with appropriate models obtained from laboratory studies, CSEM measurements can help quantify the saturation and total volume of hydrate within a known or suspected deposit. By adding geometrical constraints obtained from seismic reflection data, tradeoffs between total volume and peak saturation can be resolved. Currently our laboratory models are limited to pure hydrate and hydrate+sand. It is important to expand this library to include silt and fluids to the list. Basic data such as these will also improve the interpretation of resistivity well logs. A critical part of the proposed work is to use laboratory measurements to characterize and quantify changes in electrical conductivity of hydrate systems during dissociation induced by production (lowering pressure) or environmental change (increasing temperature). Combined with repeat field measurements to collect CSEM data, observed changes in conductivity can thus be interpreted in terms of changes in hydrate volume and extent. It is possible that climate- or production-induced changes in hydrate content may generate a more observable signal in electrical conductivity than in seismic properties.
By examining the role of grain size and fluids, the work proposed here will expand the application of our data to more complicated natural systems, and will help take the interpretation of well logs from a largely qualitative approach to something more quantitative. By collecting field data in locations where logging while drilling (LWD) data have already been collected, and coring data are likely to be collected in the future, we can further refine our ability to improve the interpretation of logs.
The UCSD team has now carried out 2D inversions of all CSEM lines spanning the four prospects in the Gulf of Mexico. For Walker Ridge 100 (Orca Basin), resistivity inversions have been converted to hydrate saturation and volume estimates. The team has also imaged hydrate that may be associated with a slump feature on this prospect and is currently preparing a write-up of the Walker Ridge 100 work for publication.
The USGS and LLNL team has completed a program of electrical conductivity measurements and cryogenic electron microscopy on samples of gas hydrate with added salt and now has results from five salt concentrations as well as hydrate synthesized from flash frozen seawater. The salt forms brine on the surface of the gas hydrate grains, which creates a connected electrical conduction path and increases the conductivity significantly over that of pure methane hydrate. At sufficiently low temperatures, the brine freezes to form halite or hydrohalite, and this phase boundary is evident in the electrical conductivity data. A manuscript on this work has undergone USGS internal review and is being prepared for submission to the Journal of Geophysical Research. The USGS/LLNL team is currently collecting laboratory conductivity data on hydrate mixed with silt and silt + flash frozen seawater.
Phase 1 – $366,579
Phase 2 – $116,986
Phase 3 – $49,841
Planned Total Funding– $533,406
Phase 1 – $180,484
Phase 2 – $101,616
Phase 3 – $80,399
Planned Total Funding– $362,499
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