Compressional wave phase velocity measurements during hydrate growth in partially and fully water saturated sandstone

Abstract

The compressional wave phase velocity (cP) has been measured as a function of hydrate saturation (SH) during hydrate growth in Bentheim sandstone. The Fourier spectrum signal processing technique was used to specifically obtain the phase velocity at frequency 500 kHz. Eight experiments were conducted on eight Bentheim sandstone samples, having initial water saturation (Swi) in the range 0.51–1. Based on the measurements, it is discussed how Swi might affect the hydrate formation pattern during hydrate growth. For the sample having Swi = 0.51, a clear increase is observed in the measured cP from the beginning of the hydrate formation process. In the literature, micropore models ascribe such an increase to hydrates partly forming in soft, low aspect-ratio pores. For the samples having Swi ≥ 0.68, there is an initial stage in the hydrate formation process with little or no change in the measured cP. The SH interval defining this first stage seems to increase for increasing Swi. As SH further increase, a second stage follows where cP increases. This is explained with hydrates first forming as a hydrate-water slurry before eventually solidifying and taking part of the solid frame. The results aid in understanding how the elastic properties of hydrate-bearing porous rocks change with SH and Swi.