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

Abstract

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.

Laboratory Insights Into the Effect of Sediment‐Hosted Methane Hydrate Morphology on Elastic Wave Velocity From Time‐Lapse 4‐D Synchrotron X‐Ray Computed Tomography

“…In general, the presence of hydrate increases the seismic velocity (Helgerud et al, 1999) and electrical resistivity (Edwards, 1997) of host sediments; this depends on the amount of hydrate occupying the pore space (saturation) and hydrate morphology, that is, spatial distribution of the hydrate grains within the host sediment (e.g., Dai et al, 2012;Ecker et al, 2000;Priest et al, 2005;Waite et al, 2009). In particular, geophysical remote sensing methods use elastic wave velocity and electrical resistivity anomalies to quantify hydrates in marine sediments, based on rock physics models that relate these anomalies to hydrate content (e.g., Collett, 2001;Cook & Waite, 2018;Doveton, 2001;Ecker et al, 2000;Edwards, 1997;Helgerud et al, 1999;Spangenberg, 2001).…”

Characterization and Quantification of Gas Hydrates in the California Borderlands

“…Pore water salinity was set to be seawater values, in keeping with pore fluid salinities found in core samples at nearby ODP Site 1013 (Shipboard Scientific Party, 1997). If we consider that m could be as low as 2.4 (based on the variations seen by Cook & Waite, 2018) and that our resistivity contrast is a lower bound, we can estimate a lower bound of gas in place to be about 20 Tcf. Clearly, access to even a single well log from this basin would tighten these bound considerably.…”

Introduction to Special Issue on Gas Hydrate in Porous Media: Linking Laboratory and Field‐Scale Phenomena

“…A calibration study is also provided by Cook and Waite (), who determine hydrate saturations from electrical resistivity measurements via Archie's equation. Instead of focusing on laboratory resistivity studies on a variety of HBS (e.g., Du Frane et al, ; Lee, Santamarina, et al, ; Priegnitz et al, ; Santamarina & Ruppel, ; Spangenberg & Kulenkampff, ), Cook and Waite () use borehole logs from permafrost and deep water gas hydrate reservoirs to constrain the Archie's exponent for high‐saturation deposits. By combining compressional velocity and resistivity logs, their empirical study shows that this exponent is 2.5 ± 0.5 for high‐saturation gas hydrate reservoirs.…”

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