Seismic rock-physics modeling of the complex-morphology hydrate reservoirs and applications

Due to the complex hydrate morphologies, many of the existing models considering the single morphology cannot accurately describe the hydrate reservoirs. Moreover, the coupled effect of hydrate saturation and morphology leads to the ambiguity of elastic responses and thus the inaccuracy of hydrate saturation estimate. Concerning the manners of different types of hydrates interplaying with sediment grains, we establish the rock physics model honoring six hydrate morphologies by combining Voigt-Reuss-Hill average, SCA-DEM theory, generalized effective medium model, Wood equation, Gassmann equation, Hill's equation and Backus average. Numerical simulation results suggest that elastic properties of the host sediments containing hydrates of varying morphologies have different sensitivities to hydrate saturation, which provides a theoretical basis for hydrate morphology detection and saturation quantification. The validation of the developed modeling strategy with experimental data demonstrates that our modeling method could not only effectively capture acoustic responses of artificial hydrate-bearing samples forming with different methods, but also reveal hydrate occurrence mechanism and its evolution pattern. The application of Mallik 2L-38 well at the Meckenzie delta shows that rockphysics modeling allowing for multiple morphologies instead of single mode could improve the accuracy of hydrate saturation estimate. Finally, the hydrate saturation and the fractions of multiple morphologies are simultaneously quantified from the sonic and density logs acquired at the well AT1-MC, Nankai Trough and the Hole U1518B, Hikurangi margin. The inversion results exhibit that hydrate saturation estimates are in good agreement with those from resistivity and/or nuclear-magnetic-resonance log, as well as core data. The predicted proportions of multiple hydrate morphologies could provide reasonable interpretations of hydrate formation and distribution for the studied areas.

Automated summary

Due to the complex hydrate morphologies, existing models cannot accurately describe hydrate reservoirs. We establish a rock physics model considering multiple hydrate morphologies to improve the accuracy of hydrate saturation estimate. Numerical simulation and experimental validation show that the model can capture acoustic responses of hydrate samples and reveal hydrate occurrence mechanism and evolution pattern.