PCCT demonstration of flow rate versus pressure gradient measurements for determining permeability in 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 the 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: Once a test specimen is isolated in the ESC (see Browse Graphic for image of the ESC in operation), 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). 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 permeability testing in the primary testing chamber of the ESC, a thin latex sleeve is pressed against the cylindrical sides of the specimen. The upper and lower specimen endcaps have porous stones connected to fluid flow lines that enable flow-through vertical permeability measurements to be made. With the latex sleeve held against the specimen by a small imposed confining stress (less than 300 kPa), fluid is restricted to flow through the specimen rather than around it. The ESC and its operations are described in Santamarina and others (2012, 2015).
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.

Effective Stress Chamber (ESC) Permeability Measurement: This dataset contains a series of flow rate versus pressure gradient measurements that, together, are used to determine the permeability of the specimen. The rate at which fluid flows from the bottom to the top of the specimen is tracked using an ISCO 500D syringe pump attached to the flow port at the porous stone on top of the specimen. The pressure difference along the specimen is measured with a Validyne DP303 differential pressure transducer through ports connected to the top and bottom endcaps. The specimen length is measured with a Linear Voltage Displacement Transducer (LVDT). The pressure gradient itself is applied by altering the pore pressure at the base of the specimen using a separate ISCO 500 D syringe pump. For each paired flow rate and pressure gradient data point pair, the flow and pressure gradient were averaged over at least a five-minute interval before shifting to a new applied pressure gradient. The results are reported here for flow rate in terms of the flow rate (meters per second) divided by the cross sectional area of the specimen for flow (square meters). For pressure gradient data, results are presented in terms of the pressure change (Pascals) divided by the sample length (meters), divided by the fluid viscosity (taken to be .0015 Pascal seconds for water). Presented in this way, as a normalized flow rate and a normalized pressure gradient, the permeability for this specimen is given directly by the slope of the best-fit straight line through the data point pairs. For this measurement at 2 Megapascal effective stress (the in situ effective stress), the result is 0.012 millidarcy (1.22x10^-17 square meters).
The measurement devices yield high-precision data that would imply a low uncertainty for the resulting permeability result. The overall uncertainty in the permeability is dominated by uncertainty in the specimen itself. Small sediment fabric alterations due to sediment disturbance during core recovery, transport or handling can create large variations in permeability. Small variations in grain size from specimen to specimen will also generate large differences in the measured permeability. In general, permeability measurements that agree within an order of magnitude are considered to be in good agreement.

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.