Evaluation of gas hydrate resources using hydrate morphology-dependent rock physics templates

Gas hydrates often exhibit several microscopic morphologies within the host sediments, which subsequently affect their elastic responses. Hence, good knowledge of hydrate morphology is essential for better understanding elastic responses of gas hydrate reservoirs and accurately estimating hydrate saturation. To detect possible hydrate morphology and quantify the hydrate saturation and other reservoir parameters (e.g., free gas saturation, porosity, clay content) directly from well logs or seismic data, we have developed a new rock physics inversion scheme based on two types of templates honoring the hydrate morphology. Through analysis of the developed rock physics templates, we found that gas hydrate reservoirs have different elastic behaviors for different hydrate morphologies. Ignoring the impact of hydrate morphology is insufficient in modeling the elastic properties of hydrate-bearing sediments and predicting hydrate saturation. Results of Mount Elbert test well at North Slope of Alaska show that hydrates are mainly presented as pore-filling form and the reservoir parameters estimated from 3D elastic template inversion are comparable to the references derived from core data or other well logging interpretations. Based on the attribute template inversion scheme, the results suggest that hydrates mainly occur as matrix-supporting or matrix-inclusion morphology and the hydrate saturation ranges from 20% to 40%, and free gas saturation varies from 0% to 10% for uniform distribution and from 10% to 35% for patchy distribution near the bottom simulating reflectors (BSR) at the Makran Accretionary Prism, Arabian Sea.