Gas-Hydrate Physical Property Research at the U.S. Geological Survey

Sampling naturally-occurring gas hydrate, and directly investigating the environments in which it forms, generally requires drilling or coring (Fig. 2 & 3). Because these operations disturb and alter the in-place, or "in situ" conditions, the Survey's physical property research program combines field and laboratory studies to provide a comprehensive framework for understanding the true in situ properties and behavior of gas hydrate-bearing sediment.  These studies include:

1. Drilling/coring naturally occurring gas hydrate-bearing sediment: Though these operations disturb the sediment, down-hole measurements generally provide the most representative estimates of the in situ physical properties.  These field projects are detailed in the USGS Energy Program web pages.
2. Near in situ, minimally-disturbed core studies: At the drilling site, pressurized cores can be taken and tested at pressure to reduce recovery-induced sample alteration.  This type of study can obtain some properties that cannot be measured down-hole.
3. Laboratory studies of recovered core material: Hydrate-bearing core can be preserved in pressure chambers or in liquid nitrogen for shipment to the specialized laboratory facilities required for measuring certain physical properties.
4. Laboratory studies of samples containing laboratory-formed gas hydrate: These studies avoid recovery and sample-handling alteration effects, and can provide calibrations and insight for interpreting property measurements on natural material.
5. Physical property measurements on hydrate-free sediment: These measurements, some of which can be made in the field (Fig. 4), provide a baseline for assessing how the presence of hydrate alters the properties and behavior of the host sediment.

These web pages provide an overview of USGS physical property research on gas hydrate, summarize key results and discuss our approach to research challenges that lay ahead. The specific capabilities and investigations at the USGS's physical property laboratories in Menlo Park, CA, Reston, VA, and Woods Hole, MA are summarized in Table 1.

Research Highlights

Pure Gas Hydrate

• Synthesis of pure hydrate:  The "seed ice" technique for forming pure gas hydrates suitable for physical property testing garnered the American Chemical Society's 1997 R.A. Glenn Award, and a cover photograph in Science.
• Self preservation: Discovered an ultra-slow hydrate breakdown rate for methane hydrate depressurized to room pressure between ‑2 and ‑30°C.  This quality is now being explored as a means of temporarily preserving gas hydrate in the field, or for transporting gas hydrate.
• Acoustic wave speeds: Obtained the first measurement of shear wave speed, and the first simultaneous measurements of shear and compressional wave speed in methane gas hydrate.
• Thermal Properties: Obtained the first simultaneous measurements of thermal conductivity, thermal diffusivity and specific heat in methane gas hydrate.
• Solubility: Obtained the first dissolution rate measurements for pure methane gas hydrate at the seafloor (in collaboration with the Monterey Bay Aquarium Research Institute), and measured the pressure and temperature dependence of methane solubility in the presence of hydrate.

Gas Hydrate Bearing Sediment (GHBS)

• First triaxial tests performed on GHBS showed a physical property dependence on the pore-space hydrate distribution. Acoustic and shear strength tests of laboratory-formed GHBS and of GHBS recovered from the Mallik 2L-38 permafrost gas hydrate site demonstrate how the hydrate formation process itself controls the properties of GHBS.
• Cryogenic Scanning Electron Microscope (CSEM): Pioneered many of the techniques req uired for imaging gas hydrate.  CSEM provides a means to visualize pore-scale gas hydrate, reveals differences between laboratory and recovered natural core, and enables the cataloging of micro-scale recovery and handling effects (Fig. 5).
• Bulk physical property changes due to handling effects: X-Ray Computed Tomography (CT) scans, combined with mechanical and thermal property measurements demonstrate how gas hydrate can redistribute within a sample during the brief depressurization often required in the recovery and transport of hydrate-bearing core material.  This process alters the measured properties of the recovered material (work done in collaboration with Dr. Kneafsey, Lawrence Berkeley National Laboratory).

The Challenge of Measuring Physical Properties on Gas Hydrate

As implied in the research descriptions and highlights above, a key laboratory challenge is to produce analytical results that are representative of the natural environment.  This goal can be achieved either by (1) bringing field samples to the laboratory or (2) reproducing naturally-occurring hydrate in the laboratory.  Both approaches require judgment to apply results to in situ conditions:

1. Generally, natural sediment undergoes some period of depressurization during the recovery process prior to being tested for physical properties.  Destabilizing hydrate-bearing sediment even briefly changes the proportions and distribution of hydrate and gas in the sample (Fig. 5), and disturbs the host sediment (Fig. 6).  These processes can significantly alter the measured properties of the sample.
   
   

   Figure 6.  Sediment core recovered offshore India.  The core was recovered in a pressure core system, shipped at pressure to the lab, but then depressurized and photographed prior to being tested in the Gas Hydrate and Sediment Test Laboratory Instrument (GHASTLI).  Fracturing of the core likely resulted from gas expansion during depressurization (Photo by William Winters, USGS).

2. Hydrate-bearing sediment formed in the laboratory can be tested without exposing the sample to hydrate-destabilizing conditions, but the samples themselves do not necessarily have the same physical properties as the natural samples they are intended to mimic.  As an example, methane hydrate formed in the laboratory in the presence of free gas generally forms as a cement that strongly binds sediment grains together.  Acoustic wave speed results, for instance, are significantly higher in this cemented, laboratory-formed material than in naturally-occurring hydrate-bearing sediment, which tends to form in a much weaker arrangement in which hydrate merely contacts grains.

Primary Research Directions

The USGS's laboratory capabilities cover a broad range of physical properties and processes (see the Laboratory Capabilities Table), but applying laboratory measurement results to natural systems requires measurements on in situ, undisturbed material.  The research directions described here are designed to approach this measurement ideal by characterizing the sample material, forming hydrate that better mimics in situ behavior, and minimizing sample disturbance on recovered core material.

1. Characterizing the hydrate, the sediment, and the combined hydrate + sediment material - Geologic and engineering models of hydrate-bearing sediment depend either explicitly or implicitly on the sediment structure in the absence of gas hydrate, and on how gas hydrate is distributed within the sediment (Fig. 7).  To effectively link our laboratory and field - derived property measurements, and to ensure our laboratory-formed samples are an appropriate analog for naturally-occurring gas hydrate-bearing sediment, sample material must be accurately characterized.  Building on over a decade of research, the laboratory characterization capabilities now encompass:
   1. The sediment: what are the sediment grains made of, how dense and how large are the sediment grains, what is the bulk porosity, pore-size distribution, pore-water content and salinity, and what is the in situ state of stress and shear strength of the sediment?
   2. The gas hydrate: what type of gas hydrate is present, how dense and how large are the hydrate grains?
   3. The combined hydrate-bearing sediment: how is gas hydrate distributed in the pore space, and how did it form?
2. Measurements on laboratory-formed methane hydrate-bearing sediment grown directly in a physical property measurement chamber - Measurements on methane hydrate-bearing sediment formed and measured in a single laboratory pressure vessel under controlled conditions provide calibrations for interpreting property measurements in natural material, and can be used to identify the primary variables controlling the behavior of natural material. By forming the methane hydrate in sediment directly in the physical property measurement vessel, sample alterations caused by recovery and sample transfer processes are eliminated. Ideally, the pore-space hydrate distribution is the same in the natural samples and their laboratory analogs, but this requires developing a laboratory hydrate formation technique that produces hydrate in contact with, but not cementing, sediment grains.  Several potential techniques are being pursued in the hydrate community, but the USGS focuses on two: i) forming methane hydrate from methane dissolved in water (Fig. 8), and ii) forming methane hydrate from ice or partially-water saturated sediment, then flooding the sample with water and allowing the hydrate to anneal to the textures observed in natural samples.  Once a reliable formation technique is developed, a full study of physical property measurements can commence.
3. Measurements on laboratory-formed methane hydrate-bearing sediment grown directly in a physical property measurement chamber - Measurements on methane hydrate-bearing sediment formed and measured in a single laboratory pressure vessel under controlled conditions provide calibrations for interpreting property measurements in natural material, and can be used to identify the primary variables controlling the behavior of natural material. By forming the methane hydrate in sediment directly in the physical property measurement vessel, sample alterations caused by recovery and sample transfer processes are eliminated. Ideally, the pore-space hydrate distribution is the same in the natural samples and their laboratory analogs, but this requires developing a laboratory hydrate formation technique that produces hydrate in contact with, but not cementing, sediment grains.  Several potential techniques are being pursued in the hydrate community, but the USGS focuses on two: i) forming methane hydrate from methane dissolved in water (Fig. 8), and ii) forming methane hydrate from ice or partially-water saturated sediment, then flooding the sample with water and allowing the hydrate to anneal to the textures observed in natural samples.  Once a reliable formation technique is developed, a full study of physical property measurements can commence.
4. Measurements on minimally-disturbed hydrate-bearing sediment - Relaxing the in situ state of stress on recovered material is a primary cause of property-altering sample disturbance.  Researchers at Georgia Tech have developed an "instrumented pressure testing chamber" (IPTC) with the capacity to accept samples from a pressure coring system without losing the original pore pressure.  Avoiding depressurization can prevent hydrate from dissociating and altering the core material prior to obtaining physical property measurements.  This task seeks to bring this technology to future USGS field programs.  The IPTC provides a suite of physical property data on the least disturbed material currently available, including strength and thermal conductivity data that are not routinely measured with current borehole technologies, and provides a critical step toward making measurements on samples for which not only the pore pressure, but the complete in situ state of stress on the gas hydrate, sediment and pore space is preserved.

Collaborators

This work has benefited tremendously from collaborations with national and international partners:

National

• Colorado School of Mines
• Department of Energy
• Georgia Institute of Technology
• Lawrence Berkeley National Laboratory
• Lawrence Livermore National Laboratory
• Marine Desalinization Systems, L.L.C.
• Monterey Bay Aquarium Research Institute
• Massachusetts Institute of Technology
• Naval Research Laboratory
• Oregon State University
• Pennsylvania State University
• Rice University
• Schlumberger, Ltd.
• Scripps Institute of Oceanography
• Stanford University
• Texas A&M University
• University of New Hampshire
• University of Southern Mississippi
• Woods Hole Oceanographic Institution

International

• Geological Survey of Canada
• Geotek, Ltd.
• Japan Petroleum Exploration Company, LTD.
• National Gas Hydrate Program of India