|Home||Archived Sept 20, 2018||(i)|
Three USGS facilities are equipped to measure gas hydrate physical properties. Of these, the Geotechnical/Gas Hydrates Laboratory at the Woods Hole Coastal and Marine Science Center (WHCMSC) is focused on hydrate in sediment, with the goal of relating laboratory measurements to the physical properties of natural systems. We've participated in a number of field activities and cruises, both to make in situ measurements and obtain samples for laboratory studies. As discussed below, the laboratory is equipped to measure a suite of physical properties used to characterize hydrate-free and hydrate-bearing natural sediment, predict in situ behavior, and provide insight into the geologic environment from which the sediment was recovered.
|Panoramic tour of the Geotechnical/Gas Hydrates Laboratory|
WHY IS GAS HYDRATE IMPORTANT?
Gas hydrate, an ice-like crystalline solid containing high concentrations of methane and other gases is a potential source of energy, a control on sea floor stability, a hazard to conventional hydrocarbon exploration and production, and may influence the global climate. Gas hydrate occurs in a wide variety of sediment types, and the intrinsic sediment or rock properties influence the quantity, distribution, and morphology of hydrate formed. Subsequent hydrate growth profoundly influences the in situ bulk sediment properties, such as its mechanical and hydraulic behavior and its response to environmental changes such as hydrate dissociation. Therefore, the sediment properties, to a large extent, determine the viability of a particular hydrate deposit as an economic resource or its significance as a geohazard or climate-change agent.
LABORATORY TEST SYSTEMS USED TO STUDY HYDRATE-BEARING SEDIMENT
Two systems in the WHCMSC Geotechnical/Gas Hydrates Laboratory are used for studying gas hydrate. The Gas Hydrate And Sediment Test Laboratory Instrument (GHASTLI) is used to study physical interactions between methane gas hydrate and host sediment. Measured properties, including acoustic velocity and electrical resistivity, can be used to calibrate models for predicting gas hydrate concentrations in sediment pore space. Shear strength and permeability of hydrate and sediment mixtures are also measured. These properties can be used as inputs for production models used to optimize the extraction of methane from hydrate as an energy resource, or for slope stability models used to determine the geohazard potential of hydrate-bearing sediment in response to environmental change.
The Hydrates From Ice (HyFI) system is used to measure thermal conductivity, thermal diffusivity, and specific heat of various materials including ice, methane hydrate, Tetrahydrofuran (THF) hydrate, and hydrate-sediment mixtures. These properties can be used to model the heat flow required for dissociating hydrate in situ when extracting methane from hydrate as an energy resource.
GEOTECHNICAL MEASUREMENTS ON HYDRATE-FREE SEDIMENT
The lab also includes a constant-rate-of-strain (CRS) consolidation cell that can be used to estimate sediment stress history and hydraulic conductivity in the field. In addition, the magnitude and rate of one-dimensional deformation under particular loading conditions can be predicted.
A TruePath triaxial cell is used to determine intrinsic engineering properties (elastic and plastic moduli, Ko stress ratio, shear strength, strength envelope) of hydrate-free samples that can be compared to results from hydrate-bearing sediment tested in GHASTLI. Each of these systems is computer controlled to allow relatively autonomous geomechanical property measurements at low stresses (less than 3 MPa) and fluid pressures less than 1.4 MPa.
Additional geotechnical measurements carried out in the lab include index properties and Atterberg limits. Index properties, such as water content and grain density, are used for relating masses and volumes of solid grains and sediment pore contents. Atterberg limits are a measure of the sediment’s plasticity, which is dependent on grain size, clay mineralogy, and pore water chemistry. Test results are compared to existing empirical relationships to predict in situ behavior. The USGS Sedimentology Laboratory in Woods Hole is equipped to perform grain size analyses on our test samples to further classify the sediment.
|Home||Archived Sept 20, 2018|