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

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, S-h, 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 S-h profile based on compressional-wave velocity log data. Using the independently determined S-h 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 approximate to 2.5 +/- 0.5. Thus, for the coarse-grained hydrate bearing (S-h>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.