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Collett, T.S., 2000

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Bibliographic Reference

Collett, T.S., 2000, Quantitative well-log analysis of in-situ natural gas hydrates: Golden, Colorado, Colorado School of Mines, Ph.D. dissertation, 535 p., 2 vol., illust. (some color).


Gas-hydrate accumulations in onshore arctic and outer continental shelf marine environments are known to contain large volumes of natural gas. The amount of gas sequestered in gas hydrates is probably enormous, but estimates are highly speculative due to the lack of previous quantitative studies. Gas volumes that may be attributed to a gas-hydrate accumulation within a given geologic setting are dependent on a number of reservoir parameters, two of which, sediment porosity and gas-hydrate saturation, can be assessed with data obtained from downhole well-logging devices. The primary objective of this study was to develop quantitative well-log evaluation techniques, which would permit the calculation of sediment porosities and gas-hydrate saturations in gas-hydrate-bearing reservoirs. The research conducted with these devices in this study can be generally divided into three categories: (1) gas hydrate well-log response modeling, (2) laboratory quantification of gas hydrate well-log responses, and (3) field data investigation and verification. During the gas hydrate well-log response modeling phase of this study, the known and modeled well-log responses attributed to the presence of gas hydrate were reviewed and assessed. The well-log response modeling phase of this study also included the assessment of existing and the development of new gas hydrate well-log evaluation techniques used to characterize sediment porosities and gas-hydrate saturations in gas-hydrate-bearing reservoirs. The laboratory phase of this study focused on evaluating the acoustic well-log properties of gas hydrates. The field investigation portion of this thesis also contains detailed estimates of the volume of gas associated with several of the regionally mapped gas-hydrate accumulations assessed in this study. In the field verification portion of this study, it was demonstrated that the quality of the available downhole-log data was the most significant factor controlling the accurate assessment of sediment porosities and gas-hydrate saturations. Within relatively high quality (in-gauge) boreholes, conventional downhole-density logs, with appropriate corrections for the presence of gas hydrates and shales, yielded the most accurate downhole-measured porosities in gas-hydrate-bearing sediments. Sensitivity analysis in the field verification portion of this thesis also demonstrated the relative importance of selecting appropriate values for the 'reservoir' constants in the porosity and gas-hydrate saturation equations developed and tested within this study. The downhole-log-derived sediment porosities, calculated in the field verification phase of this study, were relatively high ranging from about 52 to 58 percent in most of the assessed marine gas-hydrate accumulations and from about 29 to 39 percent in the permafrost-associated gas-hydrate accumulations. (Abstract shortened by UMI.)

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