Characterization of a Sediment Core from Potential Gas-hydrate-bearing Reservoirs in the Sagavanirktok, Prince Creek, and Schrader Bluff Formations of Alaska's North Slope: Part 5—Acoustic Velocity Core Studies*
R. F. Sigal, C. Rai, C. Sondergeld, B. Spears, W. J. Ebanks, Jr., W. D. Zogg, N. Emery, G. McCardle, R. Schweizer, W. G. McLeod, J. Van Eerde, 2009. "Characterization of a Sediment Core from Potential Gas-hydrate-bearing Reservoirs in the Sagavanirktok, Prince Creek, and Schrader Bluff Formations of Alaska's North Slope: Part 5—Acoustic Velocity Core Studies", Natural Gas Hydrates—Energy Resource Potential and Associated Geologic Hazards, T. Collett, A. Johnson, C. Knapp, R. Boswell
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The Anadarko Hot Ice 1 well was cored as part of a project to study the occurrence of gas hydrate on the North Slope of Alaska. The observations and measurements made at the drill site along with the subsequent core analysis are described in five individual reports published in this Memoir. This report deals with the acoustic velocity measurements made on the recovered core.
Velocity measurements were made on the core-sample plugs from the sands recovered during both phase I and phase II coring operations at the Hot Ice 1 well. Polarized shear-wave and compressional-wave ultrasonic velocities at multiple confining stresses were measured on 1-in. (2.54-cm) horizontal plugs. The phase I unconsolidated sandstone-core samples were recovered from the permafrost zone; they were obtained frozen, and the velocities were measured in that recovered state. At 800 psi (5.5 MPa) confining stress, the median compressional velocity is 3810 m/s (12,500 ft/s). Only a very weak dependence on temperature is observed. The median shear velocity is 2170 m/s (7119 ft/s). No significant shear-wave anisotropy was observed. The trend of decreasing velocity with increasing temperature is weak. Based on nuclear magnetic resonance and resistivity measurements, these samples had liquid porosities of a few percent. These velocities are consistent with a model in which ice acts as some combination of cement and a stress-supporting matrix material.
The phase II unconsolidated sand samples were recovered from unfrozen sediment. Polarized shear and compressional velocities were measured on cleaned, dried, and brine-resaturated samples. At 800 psi (5.5 MPa), the median compressional velocity is 2040 m/s (6693 ft/s) and the shear velocity is 1080 m/s (3543 ft/s). No significant shear-wave anisotropy was observed. The sands below 1900 ft (580 m) appear to be more consolidated than the shallower sands. This is reflected in a velocity increase.
Two shale samples, one from 1800 ft (549 m) and one from 2000 ft (610 m), were selected for velocity measurement. Measurements were made on horizontal, vertical, and 45° plugs. At both depths at 800 psi (5.5 MPa), the compressional horizontal velocity was 2100 m/s (6890 ft/s) and the vertical velocity was 1900 m/s (6233 ft/s). Both components of shear-wave velocity at 800 psi (5.5 MPa) were approximately 1000 m/s (3281 ft/s).