The project goal was to develop new methodologies to characterize the physical properties of methane hydrate and hydrate sediment systems.

Westport Technology Center International - Houston, TX
University of Houston - Houston, TX

Results

Project researchers created a pressure cell for measuring acoustic velocity and resistivity on hydrate-sediment cores. They utilized the measurements for input to an existing reservoir model for evaluating possible offshore hydrate accumulations. The organization of an industry-led Advisory Board and the development of a Research Management Plan have been completed. The development of a handbook for transporting, preserving, and storing hydrate core samples brought from the field to the laboratory was completed and distributed for review by industry and researchers.

Other accomplishments include having characterized the bulk system properties for various porous media containing gas, water, and hydrates. Four media (two synthetic quartz-based systems, a Berea sandstone core, and a simulated unconsolidated Gulf of Mexico sediment) were selected, and measurements of a variety of bulk system properties for each system were completed. Prior to the formation of hydrates within these cores, baseline geomechanical, electrical, and acoustic properties of the hydrate-free cores were measured and recorded as baseline information.

Benefits of this research

This project helped enhance the understanding of the properties of methane hydrate as it forms and dissociates in natural sediments. This may help aid researchers in developing the ability to effectively locate and quantify hydrate occurrence in nature, as well as in attempting to clarify proper handling methodologies for methane hydrate-containing cores recovered during research activities.

The characteristics of hydrates are generally understood in the absence of natural sediments, but much remains to be learned regarding the behavior of hydrates under actual seafloor conditions. The models that are currently available to describe that behavior are not adequate, due either to a lack of sufficient data or to insufficient confidence in the validity of mechanisms defining hydrate formation and dissolution in sediments. This project is focused on providing those data.

The proposed project entails an industry effort to collect the necessary field data to create standard methods of data acquisition and then develop or modify a reservoir model to utilize the information. The project performer has a historical record of creating similar services for the petroleum industry.

Summary

A compilation of current best practices, developed through discussions with industry, academia, national laboratories, and government agencies, was completed and is available upon request to DOE - NETL. Experiments have been carried out to form and dissociate methane hydrates within sediment samples (defined above) of varying characteristics. These activities include measuring temperature and pressure requirements for methane hydrate formation, as well as measuring characteristics in the samples, including resistivity and seismic velocities. These data were used to evaluate characteristics of the hydrate-bearing samples, including porosity, permeability, and gas, water, and hydrate saturations. In addition, the samples were CT-scanned to provide 3-D visualization of the hydrate formation and dissociation process. These experiments were performed in Westport's pressure cells, as well as in the core holder built to hold the samples described above.

The following conclusions are offered by the performer as result of work performed under this effort:

• A novel core holder was developed that allowed application of net confining stress, CT scanning, measurement of electrical resistivity, acoustic velocity, and in situ temperature during hydrate formation and dissociation.
• During hydrate formation in porous materials acoustic velocity drops, electrical resistivity increases, x-ray absorption (CT numbers) decreases in brine-saturated regions and increases in gas-saturated regions. These measurements can be used to detect the in situ hydrate distribution.
• Core tests of hydrate formation can be conducted successfully if water is injected to maintain the high pressure during hydrate formation. When methane gas was injected to maintain the high pressure during the hydrate formation, the hydrate formed in the supply side of the core and plugged the core hydraulically disconnecting the other side of the core (Tests 3-9).
• Depressurization of hydrates within cores can be conducted successfully at a constant fluid withdrawal rate. The production of water and gas was monitored by an online densitometer (Tests 10-13). Leaks developed in several experiments with hydrate dissociation by temperature increase (Test 6).
• Hydrate depressurization experiments can be matched qualitatively by the hydrate simulator (simulations of Tests 10-13). A series of simulations were run to mimic the variable pressure conditions at the production well. The simulator was able to apply constant pressure boundary condition, but not the constant total volumetric flow rate boundary condition.
• The core temperature falls temporarily during hydrate dissociation (and increases temporarily during hydrate formation). The temperature drop is higher if the fluid withdrawal rate is higher (Tests 10-13). This is due to the heat of hydrate formation. This heat is supplied to the dissociation front from the sensible heat, which lowers the temperature of the porous medium in the vicinity.
• The sodium iodide concentration affects dissociation pressure and rate.
• The pressure drops (flow effects) are small within the core. Very small pressure gradients were observed in most of the experiments except when the cores became plugged. The flow effects are expected to be larger in the field scale and may control hydrate dissociation.
• At the core scale, hydrate depressurization is kinetically controlled, because pressure drops are small. Changes in the kinetic rate constant affect the rate of methane production. At the field scale, hydrate depressurization may be controlled by other factors such as flow effects and heat transfer.

(February 2007)
All research under this award is complete. The project final report and other available deliverables are listed under the "Additional Information" section below.

$817,952

Funding
This project was awarded funding under a DOE solicitation conducted in accordance with the Methane Hydrate Research and Development Act of 2000.

$204,488

NETL – Rick Baker (Richard.Baker@netl.doe.gov or 304-285-4714)
Westport Technology Center –  John Shillinglaw (john.shillinglaw@intertek.com or 713-479-8455)

Final Report [PDF-9.04MB]

Users Manual for Natural Gas Hydrate Core Handling Procedures. CDROM