A study of electrical and thermal properties and their use to detect natural gas hydrates in ocean sediments

Abstract

Natural gas hydrates may exist in deep ocean sediments in the presence of gas. Since recovery of hydrate samples is very difficult using conventional geotechnical sampling operations the development of in situ detection methods becomes essential. In this research the electrical resistivity and thermal conductivity of hydrates were studied experimentally using tetrahydrofuran as a hydrate former. An experimental apparatus was designed for this purpose and an automated control system was developed to form cylindrical specimens, freezing them unidirectionally from top to bottom. Massive hydrates and sediment-hydrate mixtures were formed and studied. The electrical resistivity of all hydrate specimens increased drastically upon hydrate formation to values that varied from 2x10^5 to 2x10^4 Ohm-m for frequencies ranging from 100 to 1000 Hz. The presence of brine in the specimens reduced the electrical resistivity and masked the frequency effects. The thermal conductivity was determined for steady state conditions in the cylindrical specimens by solving the three dimensional axisymmetric heat conduction problem and inverting the temperature data. The thermal conductivity of massive hydrate specimens varied from 0.18 to 0.48 Wm^-1K^-1 Sand- and clay-hydrate specimens had a thermal conductivity similar to water saturated specimens, increasing with decreasing porosity. An electrical resistivity probe appears to be a promising method of hydrate detection in ocean sediments where the hydrate layer may be represented by a resistive layer in a conductive earth. Based on the studies conducted the resolution of the resistivity and thickness is very satisfactory. Hydrate formation in sediments was modelled by a moving heat source and the change of sediment temperatures due to hydrate formation was calculated based on the rate of hydrate formation and the thermal properties of the sediments. The results indicate that local changes of the geothermal gradient in conjunction with other measurements may be used to detect hydrates.