Pore Fractal Characteristics of Hydrate‐Bearing Sands and Implications to the Saturated Water Permeability

Abstract

Permeability mainly governs fluid flow through hydrate‐bearing sediments, and its theoretical models play a primary role in the efficiency prediction of gas recovery from hydrate reservoirs by using numerical simulators. Most of these numerical simulators rely on empirical or semiempirical permeability models largely due to the lack of suitable parameters that well quantify the evolution of pore structures. In this study, X‐ray computed tomography scans are conducted on methane hydrate‐bearing sands, followed by extractions of the maximal diameter and fractal dimensions for the pore space occupied by fluids. These extracted parameters, including area and volume pore‐size fractal dimensions, tortuosity fractal dimension, and the area maximal pore diameter, are further extended to develop fractal theory‐based models for predictions of the hydraulic tortuosity and the saturated water permeability in hydrate‐bearing sands. Results show that the pore space occupied by fluids within hydrate‐bearing sands is inherently fractal. The area pore‐size fractal dimension decreases but the tortuosity fractal dimension changes little with increasing hydrate saturation, and the sum of these two fractal dimensions can be used to predict the volume pore‐size fractal dimension. In addition, the area maximal pore diameter decreases with increasing hydrate saturation, and a semiempirical model is proposed. The fractal theory‐based permeability reduction model agrees well with available experimental data, and it can capture the essential physics of saturated water permeability reduction in hydrate‐bearing sediments during hydrate formation.

Maximum Sizes of Fluid-Occupied Pores within Hydrate-Bearing Porous Media Composed of Different Host Particles

“…Þ/2 when S h > 0 in a general trend (the blue dot curve in Figure 4(d)), and normalized maximum hydraulic diameter can be generally depicted by using Λ h [23,37] is applied to fit values of different normalized maximum pore diameters, and values of empirical parameters are summarized in Figure 4…”

“…These diverse pore structures are experimentally observed, and how they change during hydrate formation or dissociation has been quantified by using varieties of parameters with clear physical significances. Examples include porosity, shape factor, Euler characteristic of individual hydrate cluster, fractal dimensions, pore surface, and pore volume and size [21][22][23][24]. Various pore sizes (e.g., the critical, mean, and maximum pore sizes) have been correlated to the hydraulic permeability of porous media, and larger pore sizes generally lead to higher values of the hydraulic permeability [25][26][27].…”

“…Grain sizes of marine sediments hosting natural gas hydrates are of a wide range from clays and silts to coarsegrained sands, and sand particle shapes are generally different [34][35][36]. Effects of host sediments properties (e.g., porosity, grain size, and shape) on the maximum size of fluidoccupied pores within hydrate-bearing sediments remain elusive, although papers have been published to clarify how the maximum size of fluid-occupied pores evolve with hydrate saturation during hydrate formation and dissociation [23,37].…”

Permeability Models of Hydrate-Bearing Sediments: A Comprehensive Review with Focus on Normalized Permeability

“…As known, where and how a hydrate forms in pores significantly influence the microscopic pore structure characteristics of sediments and thus affect the permeability evolution [7]. Therefore, it is generally believed that the empirical parameter values of normalized permeability models are mostly determined by the microscopic pore structure of sediments [61]. However, the pore structure becomes more complex due to the presence of hydrates, which makes it difficult to select values of empirical parameters.…”

“…However, the pore structure becomes more complex due to the presence of hydrates, which makes it difficult to select values of empirical parameters. Hence, the fractal geometry method was adopted to study the relationship between permeability evolution and pore structure change in HBSs induced by hydrate phase transition [60][61][62] by fractal dimensions, which can quantify how inner microstructures evolve. In addition, Hou et al [63] studied the influence of hydrate pore habits and morphology on the size and tortuosity evolution of controlling seepage channels through 2D lattice Boltzmann micro-flow simulation.…”