Gas hydrate characterization in sediments via x-ray microcomputed tomography

Natural gas hydrates (NGHs) are efficient and promising energy resources because of their high energy density. In addition, NGH occurs in sediments under certain pressure and temperature conditions and has the potential to meet the increasing global energy demand. However, efficient exploitation of NGH requires a precise characterization and understanding of the hydrate formation, accumulation, and dissociation mechanisms. In this context, the microstructural characterization of gas hydrate is essential and requires specialized methods and equipment. While traditional imaging and characterization tools offer fundamental microstructural analysis, x-ray microcomputed tomography (μCT) has gained recent attention in producing high-resolution three-dimensional images of the pore structure and habits of hydrate-bearing sediments and providing the spatial distribution and morphology of gas hydrate. Further, μCT offers the direct visualization of the hydrate structure and growth habits at a high resolution ranging from the macro- to micro-metric scale; therefore, it is extensively used in NGH characterization. This review summarizes the theoretical basis of μCT imaging spanning the setup of the experimental apparatus and visualization techniques. The applications of μCT in NGH reservoir characterization, such as hydrate types and their constituents, physical and chemical properties, occurrence, and accumulation, are presented. Hydrate characterization using μCT imaging is explicitly discussed, including a general understanding of hydrate pore-habit prediction, saturation and percolation behavior, seepage and permeability, and the influence of hydrate saturation on the mechanical properties of hydrate-bearing sediments. Last, conclusions and recommendations for future research are provided. This review offers a reference for understanding the application of μCT to evaluate gas hydrates, which contributes to exploiting these energy resources.