PCCT measurements of stress and strain during direct shear tests of fine-grained sediment collected from Area C, Krishna-Godavari Basin during India's National Gas Hydrate Program, NGHP-02

Deployment sample collection: This study used sediment from pressure core NGHP-02-08-30P, collected from Site NGHP-02-08 in Area C of the Bay of Bengal offshore Eastern India. Pressure core collection requires the rotary corer to retract the core through a ball valve and into an autoclave at the in situ coring depth. Once retracted into the autoclave, the ball valve at the base of the autoclave can be closed, sealing the core within the autoclave. A connected high-pressure nitrogen canister provides pressure stabilization as the autoclave is brought through different thermal regimes on the trip to the rig floor. Once the autoclave is recovered, it is chilled in an ice bath to stabilize the core contents, then transferred to a temperature-controlled unit for core manipulation. During NGHP-02, depth below sea floor was based on the continuous downhole log of the drill pipe length. The sea floor depth reference (mudline) was determined from a combination of noting when the drill string contacted the sea floor and increased the measured weight-on-bit, and visual verification of the drill bit position using an ROV. Here, depth is reported using the IODP standard depth terminology CSF-B (total depth from sea floor to target sediment, after all gas expansion gaps have been removed).

Core preparation and transport: Note that throughout the core preparation, transport and testing stages, the hydrostatic pressure is maintained at or above in situ values to maintain the stability of the core contents and avoid the formation of gas bubbles. Once the core-filled autoclave was brought to the rig floor and its temperature was stabilized, the autoclave was transferred to a shipboard analysis laboratory. In the laboratory, a pressurized system extracted the core from the autoclave, cut the core to a prescribed length (1.2 meters for the NGHP-02-08-30P core described here), and inserted the 1.2 m-long section into a pressurized storage chamber. The storage chamber can then be isolated via a ball valve closure and, ultimately, shipped in a Department of Transportation-approved, refrigerated overpack system to the U.S. Geological Survey’s Woods Hole Coastal and Marine Science Center for analysis (WHCMSC).

Pressure Core Characterization Tool (PCCT) Core Manipulation: Once the core storage chamber arrived at WHCMSC, it was moved into a refrigerated core storage facility to maintain the core’s temperature stability. The core was subsequently tested in the WHCMSC High Pressure Core Analysis Laboratory (HyPrCAL) using the PCCTs (see the browse graphic for this data releases primary landing page). Similar to the shipboard setup, the core was first retrieved, at pressure, from the storage chamber, then individual specimens were cut and inserted into either the Direct Shear Chamber (DSC) or Effective Stress Chamber (ESC). The core manipulation and transfer into the testing chambers was accomplished at a hydrostatic pressure between 10-11 MPa (Megapascal).

Testing Chamber Overview: The DSC and its operations are described in Santamarina and others (2012, 2015). Briefly, once the specimen is isolated in the Direct Shear Chamber (DSC, see Browse Graphic), the sediment is extruded from the plastic core liner using a plunger that will eventually serve as the specimen’s top endcap once the specimen is pushed all the way out of the liner and into the primary testing space. The plunger is then used to apply a vertical effective stress, returning the specimen to its in situ state of effective stress (approximately 2 MPa). Additional vertical effective stress can be applied to test the specimen response to the increasing effective stress that will occur in situ when the formation is depressurized to extract methane from gas hydrate. The consolidation data measured while applying the full range of vertical stresses are provided in a separate data release within this larger work (see "PCCT measurements of the consolidation characteristics, constrained modulus and compressional wave velocity for fine-grained sediment collected from Area C, Krishna-Godavari Basin during India's National Gas Hydrate Program, NGHP-02" in Jang and others, (2018)). For the shear strength testing, a core specimen in the DSC is held in a three-layer specimen chamber. The middle layer can be horizontally moved, shearing the middle third of the approximately 15 cm-tall specimen along the top and bottom of the middle layer. In this data release, the following parameters are reported as functions of the applied effective vertical stress: the horizontal and vertical strain, and the ratio of shear stress to effective vertical stress. The strains (unitless) are measured using Linear Voltage Displacement Transducers, and are reported here with an accuracy of 1e(-8). Stresses are calculated from load cell measurements, and are reported here with an accuracy of 5 kPa (kilopascal).
Jang, J., Dai, S., Yoneda, J., Waite, W.F., Collett T.S., and Kumar, P., 2018, Pressure core characterization tool measurements of compressibility, permeability, and shear strength of fine-grained sediment collected from Area C, Krishna-Godavari Basin, during India's National Gas Hydrate Program Expedition NGHP-02: U.S. Geological Survey data release, https://doi.org/10.5066/P91XJ7DP.
Santamarina, J.C., Dai, S., Jang, J., and Terzariol, M., 2012, Pressure core characterization tools for hydrate-bearing sediments. Scientific Drilling, v. 14, p. 44-48.
Santamarina, J.C., Dai, S., Terzariol, M., Jang, J., Waite, W.F., Winters, W.J., Nagao, J., Yoneda, J., Konno, Y., Fujii, T., and Suzuki, K., 2015, Hydro-bio-geomechanical properties of hydrate-bearing sediments from Nankai Trough. Marine and Petroleum Geology, v. 66, p. 434-450.

Direct Shear Chamber (DSC) direct shear measurement: To obtain the stress and strain data provided in this data release, a direct shear test is performed in the following fashion: A target vertical effective stress is applied to the specimen, and held until the specimen height becomes constant, meaning the specimen has finished consolidating to the extent required for that imposed vertical effective stress. A horizontal shear is induced by hydraulically forcing the ring containing the middle third of the specimen to slide horizontally at 1.9 millimeters per minute. The horizontal and vertical stresses and strains are tracked during this shearing. Once the middle ring has been displaced by 10 millimeters, the test is considered complete. The ring is pushed back into its original position, and a new vertical load is applied to the specimen. With the increased load, the specimen consolidates, and the shear planes are shifted below the upper and lower limits of the shearing ring. A subsequent shear test will therefore shear the specimen along a new pair of horizontal shear planes.

Data archiving: Microsoft Excel version 15.33 was used to consolidate all data in a spreadsheet. Measured interface heights and elapsed times were arranged by sediment and pore fluid type. Results were then exported to a comma-separated values (csv) file format.

Added keywords section with USGS persistent identifier as theme keyword.