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 3—Electrical Resistivity 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 3—Electrical Resistivity 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 electrical resistivity measurements made on the recovered core.
The measurement and analysis of core electrical resistivities followed different procedures depending on the sample type. The sandstone samples recovered in phase II were from unfrozen sediments. These samples were processed after cleaning and drying and resaturating with brine. Porosity and formation factor were measured as a function of confining pressure. Shale samples recovered from this section of the corehole were measured at their recovered states. The phase I recovered sandstone samples came from the permafrost zone. They were first measured at below-freezing-temperature conditions with the pore fluids contained on recovery.
The phase II recovered sandstones were saturated with a 3% KCl solution, and resistivity was measured at 20°C (68°F) at confining stresses ranging from 600 to 2200 psi (4.1 to 15.2 MPa). The cementation factor extracted from the core resistivity measurements had a median value of 1.94 at 800 psi (5.5 MPa) and 1.87 at 1800 psi (12.4 MPa). Two sand-core samples were also measured as recovered to estimate the salinity of the pore water in the phase II recovered cores. These estimates were 5400 and 13000 ppm.
Two shale samples from the phase II recovered cores were measured in their recovered state. Within this study, both horizontal and vertical resistivities were measured. The horizontal sample measurements had resistivities of 3.26 and 3.25 ohm m, respectively. The corresponding vertical resistivity values done on companion plugs were 6.02 and 7.57 ohm m.
A resistivity analysis of the phase I recovered samples required assumptions and approximations because neither the percentage of unfrozen brine nor its salinity was known. The basic approximation made was that, for a plug containing both ice and unfrozen brine, the unfrozen brine has a salinity that is the lowest value needed to keep it from freezing at that temperature. We also assumed that the frozen samples satisfied Archie's Law with a cementation factor of 2. These assumptions, along with the resistivity measurements, were used to estimate the unfrozen fluid porosity and the salinity before freezing of the pore fluid. With these assumptions, the estimated salinity of the brine in the pore space before the formation of permafrost had a median salinity of 7100 ppm. The percentage of the pore space filled with brine is a function of temperature. The measurements ranged from 5% of the pore space unfrozen at −7.5°C (18.5°F) to 55% unfrozen at −1.5°C (29.3°F).