A unified effective medium modeling framework for quantitative characterization of hydrate reservoirs

Abstract

Quantitative characterization of hydrate reservoirs is essential for evaluating hydrate resources and monitoring environmental hazards. Rock-physics models provide the theoretical basis for linking the hydrate saturation and distribution to the elastic properties. However, many of the existing models ignore the coupled effect of hydrate saturation and complex morphologies on the elastic behaviors and thereby fail to accurately describe hydrate reservoirs. To address these problems, the authors have proposed a unified effective medium modeling framework honoring multiple hydrate morphologies. In this framework, the manners in which the different types of hydrates interact with the sediment grains are characterized by different effective medium models. In addition to the hydrate saturation, the modeling results also reveal the effects of the cement factor, friction coefficient, incidence angle, lithology, and hydrate morphology on the elastic properties of the hydrate reservoirs. Then, five experimental data sets for synthetic methane and tetrahydrofuran hydrates formed under different conditions are used to validate the feasibility of the unified rock-physics modeling scheme. The good agreement between the theoretical predictions and laboratory data indicates that this method can effectively determine the hydrate occurrence mechanisms from the acoustic responses in different hydrate formation environments. On the basis of the constructed elastic rock-physics templates, multiple hydrate morphologies are determined with cluster analysis and then used for constraining the modeling scheme. Finally, the proposed modeling method is tested using sonic logs from Ignik Sikumi, North Slope of Alaska, and Hole U1518B, the northern Hikurangi margin, offshore New Zealand. The hydrate saturations estimated from the velocities are consistent with the interpretations based on resistivity and nuclear magnetic resonance logs, as well as chloride measurements, confirming the applicability and feasibility of our unified theoretical modeling framework.