Vertical Seismic Profiling and seismic properties of Gas Hydrate in an Arctic well
Yanpeng Mi, Akio Sakai, Rakesh Walia, Roy D. Hyndman, Scott R. Dallimore
Gas hydrate is an ice-like nonstochiometric inclusion compound with water molecules forming a three-dimensional network within which small gas molecules (guest) can be trapped. In a collaborative Japan-Canada research project, a 1150 m research well was drilled in the Canadian Arctic to investigate gas hydrates beneath permafrost. The well located in the Mackenzie Delta, N.W.T. was part of a Japanese government-industry program to assess the potential of gas hydrates as an energy source. The primary objectives of the well were to evaluate drilling, coring and geophysical technologies prior to gas hydrate drilling program offshore Japan. The project involved comprehensive downhole measurements and laboratory studies on recovered cuttings and core. This article reports the results of vertical seismic profiling surveys (VSP), which have been used along with downhole log data, to evaluate the effect of gas hydrate on velocity and to estimate gas hydrate concentrations. VSP was recorded for both vertical and offset source positions, using multi-component receiver tools and multi-polarized vibrators. The excellent data quality allows accurate compressional and shear wave velocity profiling, as well as reliable estimation of the gas hydrate effect on sediment elastic properties. Major reflectors, such as the base of the permafrost zone and the hydrate host strata, were clearly seen in the VSP data. Corridor stack of zero-offset P-wave data and VSP-CDP transform of offset data provide good comparisons with the surface seismic data. Velocities in the permafrost section above 620 m are markedly increased by ice bonding and are generally over 2500 m/s. In the largely unfrozen section from 620 m to 890 m, the velocities are lower, and range from 2000 m/s to 2400 m/s. In the main gas hydrate zone below 890 m to the base of the hydrate stability zone at around 1100 m, velocities are increased by hydrate cementation up to 2500-3700 m/s. The estimated hydrate saturations are highly variable, reaching as high as 60% in short sections, with an average of about 20% (~7% of sediment volume for 35% porosity) in the 900-1100 m interval. Poisson's Ratio is ~0.39 in both the permafrost and gas hydrate sections compared to ~0.44 in the unfrozen sections. The seismic properties indicate that the hydrate is located mainly in the sediment pores, rather than cementing the grain contacts, and that commonly used velocity versus hydrate concentration relations are approximately valid. Gas hydrate stiffen the sediment matrix and should cause less seismic attenuation effect, hence a higher seismic quality factor (Q) than that of normal sediment. The power ratio method gave a Q of about 30 in the dominant bandwidth from 50 Hz to 100 Hz, while higher Q value for higher frequency within the normal sediment. The destructive interbed multiples caused by hydrate layers prevented even a rough Q value estimation.