Laboratory ultrasonic and resistivity measurements on sedimentary rocks containing tetrahydrofuran hydrates

In this paper, we report laboratory ultrasonic and resistivity measurements on Berea Sandstone and Austin Chalk samples saturated with a stoichiometric mixture of tetrahydrofuran (THF) and water. We used THF as the guest species rather than methane or propane gas because THF can be mixed with water to form a solution containing the proper stoichiometric proportions of the THF and water. Neither methane nor propane is soluble in water. Because THF solutions form hydrates readily at atmospheric pressure, it is an excellent experimental analogue to natural gas hydrates. Hydrate formation increased the ultrasonic P-wave velocities from a room temperature value of 2.5 km/s to 4.5 km/s at -5/sup 0/C when the pores were nearly filled with hydrates. However, lowering the temperature below -5/sup 0/C did not appreciably change the velocity. In contrast, the electrical resistivity increased nearly two orders of magnitude upon hydrate formation and continued to increase more slowly as the temperature was further decreased. In all cases the resistivities were nearly frequency independent to 30 kHz and the loss tangents were high, always greater than 5. The dielectric loss showed a linear decrease with frequency suggesting that ionic conduction through a brine phase dominates at all frequencies, evenmore » when the pores are nearly filled with hydrates. We find that the resistivities are strongly a function of the dissolved salt content of the pore water. Pore water salinity also influenced the sonic velocity, but this effect is much smaller and only important near the hydrate formation temperature. 11 references, 9 figures.« less