Quantification of Gas Hydrates from Well Logs and Seismic Data
Joel D. Walls1, Jack Dvorkin2, Lars B. Hubert1, and Paola Vera de Newton1
1Rock Solid Images, Houston, TX
2Stanford University, Stanford, CA
Gas hydrates are solids where gas molecules are locked inside cage-like structures of hydrogen-bonded water molecules. The physical properties of hydrates are remarkably close to those of pure ice: the P- and S-wave velocity in methane hydrate may reach 3.60 and 1.90 km/s, respectively, while its density is 0.910 g/cc. The corresponding values for ice are 3.89 and 1.97 km/s, and 0.917 g/cc, respectively. Thus, sediment with hydrate in the pore space is much more rigid than sediment filled solely by water.
One conventional geophysical approach to quantifying gas hydrates is to perform an acoustic impedance inversion on the seismic data and then establish a calibration between gas hydrate saturation and impedance from the log data. This works if the hydrate zones are thick, but if the zones are less than about ¼ wavelength in thickness, this approach gives an answer that is too pessimistic. In many cases, multiple stacked thin sands are the main potential source of hydrate resource. We have developed a method based on the concept of “cumulative attributes” that can be calibrated to provide useful estimates of total hydrate volume even in these thin sands.
In this paper we will show the results of this analysis on well logs and seismic data from a gas hydrate region in Alaska known as Milne Point. Even though this region is known to have thick massive gas hydrate sands, it also contains numerous thin sands that are sub-resolution for standard seismic inversion. We have modeled the cumulative attributes technique using the logs and will extend this analysis into the 3D seismic volume for one of these thin sands areas. Preliminary results of the modeling and 3D data analysis are very encouraging. As a result, we believe that this method will provide a substantial improvement over conventional impedance inversion.
AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery