Archie's Saturation Exponent for Natural Gas Hydrate in Coarse‐Grained Reservoirs

• Published in: Journal of geophysical research. Solid earth 2018-03, Vol.123 (3), p.2069-2089
• Main Authors: Cook, Ann E., Waite, William F.
• Format: Article
• Language: English
• Subjects: Bearing; Bulk density; Canyons; Carbon cycle; Electrical resistivity; Empirical equations; Energy resources; Energy sources; gas hydrate; Gas hydrates; Geophysics; Gulf of Mexico; hydrate saturation; Hydrates; Mallik; Mathematical models; Natural gas; Permafrost; Potential energy; Reservoirs; resistivity; Sand; Saturation; Seismic velocities; Shear strength; velocity; Wave velocity; Well logs
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1002/2017JB015138

Accurately quantifying the amount of naturally occurring gas hydrate in marine and permafrost environments is important for assessing its resource potential and understanding the role of gas hydrate in the global carbon cycle. Electrical resistivity well logs are often used to calculate gas hydrate saturations, Sh, using Archie's equation. Archie's equation, in turn, relies on an empirical saturation parameter, n. Though n = 1.9 has been measured for ice‐bearing sands and is widely used within the hydrate community, it is highly questionable if this n value is appropriate for hydrate‐bearing sands. In this work, we calibrate n for hydrate‐bearing sands from the Canadian permafrost gas hydrate research well, Mallik 5L‐38, by establishing an independent downhole Sh profile based on compressional‐wave velocity log data. Using the independently determined Sh profile and colocated electrical resistivity and bulk density logs, Archie's saturation equation is solved for n, and uncertainty is tracked throughout the iterative process. In addition to the Mallik 5L‐38 well, we also apply this method to two marine, coarse‐grained reservoirs from the northern Gulf of Mexico Gas Hydrate Joint Industry Project: Walker Ridge 313‐H and Green Canyon 955‐H. All locations yield similar results, each suggesting n ≈ 2.5 ± 0.5. Thus, for the coarse‐grained hydrate bearing (Sh > 0.4) of greatest interest as potential energy resources, we suggest that n = 2.5 ± 0.5 should be applied in Archie's equation for either marine or permafrost gas hydrate settings if independent estimates of n are not available. Key Points We develop a velocity‐based gas hydrate saturation model to calibrate Archie's saturation exponent, n We find n = 2.5 ± 0.5 applies to gas hydrate‐bearing coarse‐grained reservoirs in both marine and permafrost environments If possible, n should be calibrated using reservoir‐specific data, but if those data are not available, n = 2.5 ± 0.5 is recommended