|Measurement and Interpretation of Seismic Velocities and Attenuation in Hydrate-Bearing Sediments
||Last Reviewed November 2016
The primary project objectives are to relate seismic and acoustic velocities and attenuations to hydrate saturation and texture. The information collected will be a unique dataset in that seismic attenuation will be acquired within the seismic frequency band. The raw data, when combined with other measurements (e.g., complex resistivity, micro-focus x-ray computed tomography, etc.), will enable researchers to understand not only the interaction between mineral surfaces and gas hydrates, but also how the hydrate formation method affects the hydrate-sediment system in terms of elastic properties.
An over-arching goal of this research is to calibrate geophysical techniques for hydrate exploration, evaluation, and production monitoring. Extensive field data of hydrate-bearing sediments exist, but quantitative estimates of the amount and distribution of hydrates are difficult to determine. In addition, in situ substitution of carbon dioxide (CO2) for methane (CH4) has been proposed (and briefly tested) as a method of both extracting CH4 and sequestering CO2. Our suite of measurements can be systematically applied to both borehole and surface data to derive hydrate saturation.
Colorado School of Mines, Golden, CO 80401
United States Geological Survey (USGS), Denver, CO 80225
Differences in gas hydrate generation methods influence hydrate habit and distribution in both natural and synthetic hydrate-bearing sediments and should affect the physical properties of the hydrate. However, mechanical interactions between hydrates and sediments and the controls exerted by the hydrate formation method are poorly understood. It is critical to understand and model these effects on seismic properties, since existence and indirect mapping of hydrate-bearing formations and hydrate saturations rely heavily on indirect seismic mapping techniques. The research will help to fill this gap via a series of well-defined experiments designed to calibrate the seismic response of hydrate-bearing sediments. The experiments comprise a series of petrophysical tests on laboratory-formed hydrate-bearing sediments, including frequency-dependent measurements of seismic velocities and attenuation. Various hydrate-formation methods will be tested, including methane injection into partially water-saturated sand, circulation of methane-saturated water, and the use of tetrahydrofuran (THF) as a hydrate former. These formation methods are expected to yield “cementing” and “non-cementing” hydrates and the resulting hydrate-bearing samples will represent end-members for effective medium models of mechanical and electrical properties. Micro-focus x-ray computed tomography would provide verification and quantification of hydrate distribution in the pore space and throughout the sample. Rock physics models that link the hydrate saturation and distribution to the elastic properties of the hydrated sediment will be developed based on laboratory results. Finally, available field data, such as sonic well logs, will be inverted for attenuation as a function of frequency. The obtained attenuation data will be used in conjunction with velocity information and developed rock physics models to investigate targeted gas hydrate-bearing areas, specifically with respect to their potential formation history.
The project outcome should provide a means of calibrating geophysical field data, insight into the formation of natural gas hydrate deposits, improved ability to detect and evaluate hydrates in the subsurface and, ultimately, a method to help researchers design feasible, economic, and safe production schemes for natural gas hydrates.
Accomplishments (most recent listed first)
Low frequency experiments were successfully conducted on THF hydrate saturated rock.
THF hydrate was successfully grown in a pressure vessel.
Researchers obtained well logs from areas of known hydrate occurrence and established a petrophysical database.
Researchers have completed stability and property modeling of hydrates, including all expected pressure, temperature, and compositional conditions.
Micro CT scanner images were utilized to characterize the texture and location of hydrates forming in sediments and glass beads.
Laboratory measurements were conducted to obtain information about the distribution of hydrate in the pore space of synthetic coarse-grained sediments. THF was used as a guest molecule as THF hydrate is a proxy for methane hydrate. Micro X-ray computed tomography (MXCT) and ultrasonic velocity measurements were performed on laboratory formed glass-bead samples. Both measurements—MXCT images and ultrasonic velocity measurements—confirmed that hydrate formed out of solution within the pore space away from the grain surfaces.
The experimental laboratory equipment was modified to allow researchers to synthesize and measure the parameters of various hydrate compounds. The low frequency device was modified and updated to permit hydrate measurement and a new temperature control system was installed.
Current Status (November 2016)
Ultrasonic velocity measurements of pure sand packs and sand packs mixed with clay were analyzed with regard to their ultrasonic attenuation. The attenuation analysis shows that the clay initially causes an increase in P-wave attenuation, but after hydrate formation and dissociation, these values are comparable to the values of the measurements for pure quartz sand. This observation leads to the assumption that after hydrate formation, clay grains are either pushed closer to the quartz grains and/or the clay grains are compressed into smaller aggregates. The observed attenuation values after hydrate formation show no influence with respect to the presence of clay but shows a distinct change in the loss mechanisms. A comparison of ultrasonic attenuation measurements with well log data shows agreement between the two data sets. Seismic attenuation data are now being compiled to investigate if the loss mechanisms are independent of frequency over the full frequency range.
The micro CT pressure and temperature control system were updated by including a feed-through for fluid lines, wires, and ultrasonic P-wave transducers. The experimental setup has been pressure tested for pressures up to 5000 psi (35 MPa). The pressure and temperature control system, in combination with ultrasonic transducers, now allows researchers to form methane hydrates and to image hydrate distribution in rocks while simultaneously identifying their influence on ultrasonic velocities.
CSM plans to focus future efforts on forming and conducting experiments on methane hydrates samples. After successfully forming methane hydrates, CSM plans on injecting CO2 to perform CH4 – CO2 exchange experiments to study their effects on acoustic, elastic, and attenuation properties.
Project Start: October 1, 2012
Project End: December 31, 2016
Project Cost Information:
Phase 1 - DOE Contribution: $225,414; Performer Contribution: $65,167
Phase 2 - DOE Contribution: $215,629; Performer Contribution: $65,167
Phase 3 - DOE Contribution: $214,673; Performer Contribution: $65,167
Planned Total Funding - DOE Contribution: $655,716; Performer Contribution: $195,501
NETL – Skip Pratt (firstname.lastname@example.org or 304-285-4396)
Colorado School of Mines – Manika Prasad (email@example.com or 303-273-3457)
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