Review and Outlook of Visual and Geophysical Detections for the Pore-Scale Morphology of Gas Hydrates

The physical properties of gas hydrate-bearing sediments are closely related to the pore morphology of hydrates. According to the contact relationship and coupling strength between hydrates and sediment grains, the pore-scale morphology of gas hydrates is generally divided into pore suspending, frame supporting, and grain cementing. The studies of hydrate morphology are of great significance in evaluating gas hydrate resources. This review article reviews the main methods, new progress, existing problems, and development trends of hydrate morphology research. The research methods for hydrate morphology are mainly divided into two categories. One method is directly observing hydrate morphology and distribution characteristics via visualization methods (e.g., X-CT, MRI, microfluidics, and SEM). The other is indirectly identifying the hydrate morphology via geophysical responses (e.g., acoustic velocity and electrical resistivity) combined with rock physics models. Resistivity properties can distinguish between pore-suspending and frame-supporting hydrates at low hydrate saturation. Acoustic properties reflect the coupling strength between hydrates and sediment grains and thus confirm pore-suspending, frame-supporting, and grain-cementing hydrates at medium and high hydrate saturation. The proportions of different hydrate morphologies are unclear, dramatically increasing the uncertainty in predicting the physical properties of hydrate-bearing sediments and assessing gas hydrate resources. The joint analysis of visual observation and geophysical measurement reduces this uncertainty and provides a feasible method for quantitatively characterizing the hydrate morphology.