Dependence of Archie’s saturation exponent on hydrate saturation and hydrate morphology: a study from fluid-displacing and fracture-filling hydrate reservoirs

Accurate quantification of natural gas hydrate is essential for resource potential and climate impact assessment. Archie’s empirical equations are commonly used to quantify hydrates from electrical resistivity measurements. One dominant Archie equation parameters, that is, saturation parameter (n), is generally assumed to be constant for different hydrate saturation range for a given reservoir. However, n actually varies with hydrate saturation and morphology, and the exact relationship between n and hydrate saturation or morphology still remains poorly understood, leading to great uncertainties in resistivity-derived saturations. Here we investigate the effect of hydrate saturation and dominant hydrate morphologies on n using well logs from four sites in both fine and coarse-grained sediments: two sites with fluid-displacing hydrate (site W11 from the third Guangzhou Marine Geologic Survey; and Mallik 5L-38 well in the Mackenzie Delta) and two sites with fracture-filling hydrate (site 10 from Indian National Gas Hydrate Program Expedition 01; and site W08 from the second Guangzhou Marine Geologic Survey). We calculated n value using Archie’s law with hydrate saturation determined from velocity. Our results demonstrate a clear negative relationship between hydrate content and n value. Moreover, n estimates from two fracture-filling sites show greater variability compared to fluid-displacing sites. At a fracture-filling hydrate site, site 10, various trends between n and hydrate saturation are possibly caused by the distinct gas compositions of hydrate. Our results demonstrate that significant effects of hydrate morphology and saturation on n that are site specific, and can be used to enhance the accuracy of gas hydrate quantification.