Effect of Ice Saturation on Gas Permeability and Elastic Wave Velocity of Frozen Berea Sandstone

Abstract

In order to establish new technology for the storage of natural gas hydrates in underground rock tanks, it is necessary to precisely and quantitatively evaluate and understand the gas/liquid permeability characteristics as well as the mechanical and deformational characteristics of the rock mass at low temperatures. Moreover, based on these characteristics, it is essential to evaluate the behavior, robustness, and tightness of the rock tanks.

In this study, gas permeability experiments were conducted on Berea sandstone at low temperatures, which have not been done before, to evaluate the effect of ice saturation on the permeability. As a result, it was confirmed that the permeability tended to decrease with an increase pore fluid pressure. Compared to the initial permeability measured at 20°C, the permeability measured at −20°C showed a smaller value. Furthermore, it was observed that the permeability decreased with increasing ice saturation. The water permeability was estimated by taking into account the Klinkenberg effect, and the change in permeability was examined using the Kozeny-Carman equation. As a result, the permeability at a pore occupation ratio of 0, estimated from the regression curve, tends to overestimate the permeability obtained from the water permeability experiments.

The relationship between the P-wave velocity and ice saturation was found by measuring the elastic wave velocity, and the validity of the experimental results was discussed using estimation equations based on the cementation theory. The P-wave velocity at low temperatures tended to increase with increasing ice saturation and was 2.0 times higher when the pore occupation ratio was greater than 70%. In the range of porosity between 0.1 and 0.2, the experimental values were within the range of the estimated values, while in the range of porosity below 0.1, the experimental values exceeded the estimated values.