Pore Habit Research Articles

P-wave velocity evolution and interpretation of seepage mechanisms during CO2 hydrate formation in quartz sands: Perspective of hydrate pore habits

The hydrate-based CO2 storage (HBCS) is a promising technology for controlling CO2 emissions. A comprehensive understanding of the hydrate pore habits and seepage mechanism of hydrate-bearing sediments (HBSs) could provide a theoretical basis for HBCS. In the study, the hydrate occurrence characteristics, water migration and P-wave velocity response during hydrate formation in quartz sands are investigated. In addition, a new theoretical equation that relates P-wave velocity to permeability is presented. The digital core of porous media containing hydrate is also established, and the seepage characteristics during hydrate formation are revealed by numerical simulation. The results show that hydrates are preferentially generated in large pores and initially appear in a non-cementing pore-filling pattern, while an annular flow path for water is formed in quartz sands during hydrate formation. The hydrate morphology is changed from a pore-filling pattern to a patchy pattern at a critical hydrate saturation of around 3–7% for quartz sands with various particle diameters. The formation sites and spatial heterogeneity of hydrates can be depicted by the evolution pattern of streamlines. The results of the experiments indicate that the new semiempirical model is effective for estimating the permeability from the P-wave velocity of HBSs. These findings offer significant theoretical and practical insights for HBCS in porous media.

Mechanical behaviour and microstructure of methane hydrate-bearing sandy sediment observed at various spatial scales

Methane hydrates (MHs) are considered an alternative energy resource but also a potential source of geo-hazards and climate change. The physical/mechanical properties of gas hydrate-bearing sandy sediments are strongly dependent on the distribution of hydrates within the pore space. The purpose of this study is to investigate morphologies and pore-habits of MHs formed in sandy sediments by means of experiments that probe a wide range of scales, from the pore scale – using Synchrotron X-Ray Computed Tomography (SXRCT) and optical microscopy – to the core scale, through mechanical property measurements. The same synthetic sands are used, in which MHs are generated successively under excess gas and excess water conditions. At the macroscopic (core) scale, MH pore habits are inferred by comparing the measured sonic wave velocities to velocities calculated from rock physics models and further assessed via triaxial compression tests. Furthermore, Magnetic Resonance Imaging is used to investigate the kinetics of MH formation and distribution along the core height. The pore habits and MH morphologies are directly visualized at the pore (grain) scale by SXRCT and, with still better spatial and temporal resolution, by transmission optical microscopy, revealing some more complex morphologies than in the hydrate pore habits commonly admitted.

Brine Drying and Salt Precipitation in Porous Media: A Microfluidics Study

AbstractDrying and salt precipitation in porous media impact various engineering disciplines, including carbon geological storage in brine reservoirs, with considerable hydraulic and mechanical effects. However, understanding the underlying mechanisms and controlling factors is limited due to the lack of pore‐scale observation and quantitative analysis. We design radial flow microfluidic chips with varying pore heterogeneity and wettability and quantify the pore‐scale dynamics of brine drying and salt precipitation using time‐lapse images and image processing techniques. The gas injection process determines the distribution of residual (drying) brine and is influenced by pore heterogeneity. Higher concentrations of residual brine are observed near the injection well in more heterogeneous porous media. Capillary backflow replenishes evaporative losses near the well during drying in hydrophilic chips. Brine clusters induced by gas injection connect through brine films and reconnect by capillary backflow. Two types of pore habits of salt precipitation are identified: bulk salt within brine clusters and aggregated salt at the air‐brine interface. Salt precipitation, particularly near the injection well, significantly reduces injectivity, with less impact observed in the porous media with uniform pore structure and hydrophobic surfaces.

Three-dimensional pore-scale study of methane hydrate dissociation mechanisms based on micro-CT images

<p>Methane hydrate is a promising source of alternative energy. An in-depth understanding of the hydrate dissociation mechanism is crucial for the efficient extraction. In the present work, a comprehensive set of pore-scale numerical studies of hydrate dissociation mechanisms is presented. Pore-scale lattice Boltzmann (LB) models are proposed to simulate the multiphysics process during methane hydrate dissociation. The numerical simulations employ the actual hydrate sediment pore structure obtained by the micro-CT imaging. Experimental results of xenon hydrate dissociation are compared with the numerical simulations, indicating that the observed hydrate pore habits evolution is accurately captured by the proposed LB models. Furthermore, simulations of methane hydrate dissociation under different sediment water saturations, fluid flow rates and thermal conditions are conducted. Heat and mass transfer limitations both have significant effects on the methane hydrate dissociation rate. The bubble movement can further influence the dissociation process. Dissociation patterns can be divided into three categories, uniform, non-uniform and wormholing. The fluid flow impacts hydrate dissociation rates differently in three-dimensional real structures compared to two-dimensional idealized ones, influenced by variations in hydrate pore habits and flow properties. Finally, upscaling investigations are conducted to provide the permeability and kinetic models for the representative elementary volume (REV)-scale production forecast. Due to the difference in the hydrate pore habits and dissociation mechanisms, the three-dimensional upscaling results contrast with prior findings from two-dimensional studies. The present work provides a paradigm for pore-scale numerical simulation studies on the hydrate dissociation, which can offer theoretical guidance on efficient hydrate extraction.</p>

Investigation of particle-scale mechanical behavior of hydrate-bearing sands using DEM: Focus on hydrate habits

Natural gas hydrates are commonly found in onshore permafrost and seabed regions. Hydrates often exhibit different pore habits in different regions, which may affect the mechanical behavior of hydrate-bearing sands (HBSs). In this study, pore-filling, load-bearing and cementing HBSs were prepared using discrete element method (DEM). The numerical biaxial compression tests show that the digital cementing HBS specimen well capture the stress-strain response of the gas-saturated HBS specimen in experiments under σc = 3 MPa, which justify the DEM model in this study. The pore-filling hydrates contribute little to the stress-strain response of sediments, while the load-bearing and cementing hydrates significantly enhance the failure strength and stiffness, and also induce strain-softening behavior and higher volume dilation. Hydrate cementation hinders the lateral fabric development in HBS, and renders orderly particle displacement and abundant strong force chains. The yielding of HBS is dominated by the buckling of strong force chains. Particle sliding, dislocation and rotation are responsible for the formation and development of shear bands, which governs the volume dilation and non-uniform deformation of HBS. Comprehending the geomechanical behavior of HBS under different hydrate habits facilitates the optimization of engineering solutions for hydrate exploitation.

Theoretical analysis of shape factor of gas hydrate sediments under stress dependence

The Kozeny-Carman (KC) equation is used extensively to characterize the effective permeability of gas hydrate sediments. However, most existing modeling approaches ignore the changes in the effective stress, hydrate saturation, and pore structures of porous media, which significantly affect the extraction and exploration of gas hydrates. In addition, theoretical models that can accurately describe shape factors during the imbibition process and predict the effective permeability of gas hydrate sediments are rarely reported. This paper proposes a theoretical stress-dependent shape factor model to study the physical relations between the shape factor and relevant parameters (e.g., effective stress, hydrate saturation, effective porosity, and pore fractal dimension). Then, the derived model is extended to study the shape factor during the imbibition process. The two main novel features of the proposed model are: (1) it takes the effective stress, hydrate saturation, and pore structures into account; and (2) it can be used to study the evolution of shape factors during the imbibition process in gas hydrate sediments. The predictions from the developed model provide a good agreement with experimental results, which validates the model. In addition, based on the derived model, the effects of relevant parameters on shape factors are studied theoretically. From a practical standpoint, the model can not only estimate the effective permeability of gas hydrate sediments under stress dependence but also provide insight into the evolution of shape factor, pore structures (e.g., capillary total number, pore fractal dimension, tortuosity fractal dimension, and effective porosity) and hydrate pore habits during the extraction of hydrates.

Effective permeability changes during hydrate production

The effective permeability impacted hydrate production severely. But its changes during hydrate production are studied few, due to no stable pressure and temperature for permeability measurement. Herein, this condition was achieved by the intermittent depressurization. The effective permeability of hydrate-bearing samples was measured. The feasibility of this method was analyzed by hydrate dissociation and saturation. Afterwards, the measured and predicted effective permeability were compared and further analyzed from hydrate distribution, preferential flow path, and fresh hydrates. The results showed that hydrate dissociation of 1.24%–2.22% did not impact measurements. Hydrate distribution, preferential flow path, and fresh hydrates were found to have a larger effect on the random changes of the effective permeability during hydrate production in comparison of hydrate saturation and pore habits. The stronger heterogeneity of hydrate distribution caused the greater randomness that the effective permeability difference was 0.346 D-0.4 D at the close hydrate saturations. Under this situation, the fixed preferential flow path was possible to form and stabilized effective permeability at 0.563 D-0.569 D. In the sample with a weaker heterogeneous hydrate distribution, the more apparent randomness was increased by fresh hydrate saturation of 1.66%–5.08%. This work may be beneficial to understand gas-water flow capacity in real settings.

Multiple-Point Geostatistics Simulation of the Natural Gas Hydrate Distribution at the Pore Scale

Heat and mass transfer in a natural gas hydrate reservoir is strongly affected by the hydrate pore habit. Some present simplified models used in numerical simulation of a hydrate-bearing sediment are difficult to catch features of the hydrate distribution in sediments precisely. Using an improved multiple-point geostatistics simulation method, this study generates a series of images of porous media that reproduce the hydrate distribution information on a training image. The improved algorithm completely solves the problem that the hydrate blocks the boundary and makes the variogram and visual properties similar to the training image. Some typical phenomena of hydrate formation appear in the results, such as Ostwald ripening and the shift of hydrate pore habit from grain coating toward pore filling. The feasibility of predicting the pore habit of a hydrate with time using this novel method is proven by the verification case. This study offers a new approach to predict the distribution behavior of a hydrate, and this algorithm may be applied widely by scanning training images from different reservoirs.

Effect of hydrate distribution on effective permeability of hydrate-bearing sediments

Hydrate distribution influences the gas-water flow capacity and thus hydrate production in the hydrate-bearing sediments. The effective permeability is a key parameter to evaluate hydrate production capacity. However, few laboratory studies are focused on the effect of hydrate distribution on the effective permeability in the hydrate-bearing sediments. Thus, the water-phase effective permeability of nine hydrate-bearing samples prepared in a large-scaled pressure vessel under the excess-water setting was measured. The changes of pressure drop, temperature, and the electrical resistance during the measurements were analyzed. The measured and predicted effective permeability were compared. Afterwards, the heterogeneity of hydrate distribution was represented by the net increase of the electrical resistance (NIR) and its variation coefficient (VNIR) to define its effect on the effective permeability. The results showed that water flow channel change for the fresh hydrates caused the averaged pressure drop difference of 39.73 kPa in the repeated measurements. Hydrate dissociation for water flow impacted permeability measurements little. Moreover, the heterogeneous hydrate distribution, instead of hydrate saturation and pore habits, was found to cause a large deviation between the measured and predicted effective permeability. In the sample with hydrate saturation of 3.92%–8.01%, hydrates were mainly distributed at some local position. The corresponding effective permeability had a larger randomness because of the severer heterogeneity. Besides, as hydrate saturation was less than 19.46%, heterogeneity degree changes were opposite to the effective permeability changes. Instead, hydrate distribution impacted the effective permeability little. This work is beneficial to understand the physical properties of the hydrate-bearing sediments.

Numerical Simulation of Electrical Properties for Pore-Scale Hydrate-Bearing Sediments with Different Occurrence Patterns and Distribution Morphologies

Characterizing the electrical properties of hydrate-bearing sediments, especially resistivity, is essential for reservoir identification and saturation evaluation. The variation in electrical properties depends on the evolution of pore habits, which in turn are influenced by the hydrate growth pattern. To analyze the relationship between hydrate morphology and resistivity quantitatively, different micromorphologies of hydrates were simulated at the pore scale. This study was also conducted based on Maxwell’s equations for a constant current field. During numerical simulation, three types of hydrate occurrence patterns (grain-cementing, pore-filling and load-bearing) and five types of distribution morphologies (circle, square, square rotated by 45°, ellipse and ellipse rotated by 90°) in the pore-filling mode were considered. Moreover, the effects of porosity, the conductivity of seawater, the size of the pore-throat and other factors on resistivity are also discussed. The results show that the variation in resistivity with hydrate saturation can be broadly divided into three stages (basically no effect, slow change and rapid growth). Compared with the grain-cementing and pore-filling modes, the resistivity of the load-bearing mode was relatively high even when hydrate saturation was low. For high hydrate-saturated sediments (Sh > 0.4), the saturation exponent n in Archie equation was taken as 2.42 ± 0.2. The size of the throat is furthermore the most critical factor affecting resistivity. This work shows the potential application prospects of the fine reservoir characterization and evaluation of hydrate-bearing sediments.

Upscaling methane hydrate dissociation kinetic model during depressurisation

In the present work, a pore-scale numerical simulation of methane hydrate dissociation by depressurisation is conducted to analyze the effect of heat and mass transfer on the dissociation rate for scaling up the kinetic model at the representative element volume (REV) scale. The mass transport limitation shows that the hydrate dissociation preferred to occur near the gas phase. The effective reaction surface area is introduced to measure the exposed hydrate surface to the gas phase during gas and water migration and is modelled as a function of local hydrate and water saturation and hydrate pore habits. Heat transport limitation is computed with the one-temperature model due to the local thermal equilibrium. Compared to the pore-scale simulation, the proposed REV-scale kinetic model predicts dissociation rates with a relative error of less than 10%, which is expected to increase the precision of the hydrate recovery forecast.

Pore-scale modeling of seepage behaviors and permeability evolution in heterogeneous hydrate-bearing sediments and its implications for the permeability model

Permeability is one of the key factors determining the recovery of gas resources from natural gas hydrate (NGH) reservoirs. The heterogeneous distribution of hydrates induced by random nucleation and growth in the pore space of hydrate-bearing sediments (HBS) inevitably results in pore space structure change, thereby significantly affecting fluid flow and permeability. In this paper, the seepage behaviors in a series of two-dimensional (2D) idealized HBS models were simulated based on the pore-scale computational fluid dynamics (CFD) modeling to obtain permeability and pore-scale seepage field. The 2D hydrate heterogeneity degree coefficient (DH) was proposed to quantitatively characterize the hydrate distribution heterogeneity. The pore-throat evolution characteristics were quantified by average pore-throat size and pore-throat size distribution. The results show that the occurrence and growth of hydrates within pores lead to the reconstruction and evolution of pore space structure and the formation of new fluid migration channels. The main reason for the reduction of HBS permeability is that with the increase of hydrate saturation, pores and throats with small sizes gradually become dominant, pore space connectivity is weakened, and the tortuosity for fluid flow increases. Additionally, the evolution of flow path and flow channel morphology caused by the growth of hydrates with different pore habits leads to the difference in pore-scale seepage behaviors and permeability variations in the homogeneous model. However, the effect of hydrate pore habits on permeability is no longer obvious due to the spatial heterogeneity of sediment grains and gas hydrates in the heterogeneous model. The effective flow path for pore fluid controlled by overall hydrate distribution and pore-throat characteristics would be critical. The comparison of calculated permeability results reveals that the accuracy of reported permeability reduction models is limited due to the ignorance of the HBS heterogeneity, the actual hydrate pore habits and morphology, and the pore structure geometry. It is hoped that the findings of this study can provide some insights into the pore-scale seepage characteristics of HBS, providing important implications for the development of the permeability model that can be easily implemented in the numerical simulators for field-scale gas production optimization of hydrate reservoir.

Coupled thermo-hydro-mechanical-chemical modeling of fines migration in hydrate-bearing sediments with CFD-DEM

Fines migration associated with the multiphase flow in the exploitation of hydrate-bearing sediments (HBS) usually induces local clogging and sand production around wells, and thus its behavior with multi-field coupling is of vital importance but still poorly discovered. This paper establishes a coupled thermo-hydro-mechanical-chemical (THMC) model incorporating fines migration in HBS from micro- to macro-scale. Two typical hydrate pore habits, i.e., grain coating and pore filling, are simulated with the discrete element method (DEM) under different depressurization modes, water flow is simulated using computational fluid dynamics (CFD), and heat transfer and chemical reactions are also considered in coupled CFD-DEM simulations. Two distinct fines migration modes and their consequence on the mechanical and hydromechanical properties are revealed. For the grain-coating habit, the coarse particle size reduction induced by hydrate dissociation under an intense depressurization decreases the constriction size, increasing the local pore-clogging probability and reducing the growth rate of the hydraulic conductivity. Conversely, the fine particle size reduction in the pore-filling habit facilitates fines migration and thus sand production, with hydromechanical properties of HBS evolving oppositely compared to the clogging case.

Gas Permeability, Pore Habit, and Salinity Evolution during Methane Hydrate Dissociation in Sandy Sediments

Gas Permeability, Pore Habit, and Salinity Evolution during Methane Hydrate Dissociation in Sandy Sediments

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

Natural gas hydrates (NGHs) are regarded as a new energy resource with great potential and wide application prospects due to their tremendous reserves and low CO2 emission. Permeability, which governs the fluid flow and transport through hydrate-bearing sediments (HBSs), directly affects the fluid production from hydrate deposits. Therefore, permeability models play a significant role in the prediction and optimization of gas production from NGH reservoirs via numerical simulators. To quantitatively analyze and predict the long-term gas production performance of hydrate deposits under distinct hydrate phase behavior and saturation, it is essential to well-establish the permeability model, which can accurately capture the characteristics of permeability change during production. Recently, a wide variety of permeability models for single-phase fluid flowing sediment have been established. They typically consider the influences of hydrate saturation, hydrate pore habits, sediment pore structure, and other related factors on the hydraulic properties of hydrate sediments. However, the choice of permeability prediction models leads to substantially different predictions of gas production in numerical modeling. In this work, the most available and widely used permeability models proposed by researchers worldwide were firstly reviewed in detail. We divide them into four categories, namely the classical permeability models, reservoir simulator used models, modified permeability models, and novel permeability models, based on their theoretical basis and derivation method. In addition, the advantages and limitations of each model were discussed with suggestions provided. Finally, the challenges existing in the current research were discussed and the potential future investigation directions were proposed. This review can provide insightful guidance for understanding the modeling of fluid flow in HBSs and can be useful for developing more advanced models for accurately predicting the permeability change during hydrate resources exploitation.

Pore-scale observations of natural hydrate-bearing sediments via pressure core sub-coring and micro-CT scanning

Both intra-pore hydrate morphology and inter-pore hydrate distribution influence the physical properties of hydrate-bearing sediments, yet there has been no pore-scale observations of hydrate habit under pressure in preserved pressure core samples so far. We present for the first time a pore-scale micro-CT study of natural hydrate-bearing cores that were acquired from Green Canyon Block 955 in UT-GOM2-1 Expedition and preserved within hydrate pressure–temperature stability conditions throughout sub-sampling and imaging processes. Measured hydrate saturation in the sub-samples, taken from units expected to have in-situ saturation of 80% or more, ranges from 3 ± 1% to 56 ± 11% as interpreted from micro-CT images. Pore-scale observations of gas hydrate in the sub-samples suggest that hydrate in silty sediments at the Gulf of Mexico is pore-invasive rather than particle displacive, and hydrate particles in these natural water-saturated samples are pore-filling with no evidence of grain-coating. Hydrate can form a connected 3D network and provide mechanical support for the sediments even without cementation. The technical breakthrough to directly visualize particle-level hydrate pore habits in natural sediments reported here sheds light on future investigations of pressure- and temperature-sensitive processes including hydrate-bearing sediments, dissolved gases, and other biochemical processes in the deep-sea environment.

New insights into gas production behavior and seepage-wettability evolution during methane hydrate dissociation in sand matrix by NMR investigation

The commercial and sustainable exploitation of NGH reservoir requires a comprehensive understanding of gas production behaviors and seepage-wettability evolutions during hydrate dissociation process in hydrate bearing sediments. In this paper, the in-situ real-time CH4 hydrate dissociation process in sand pack was acquired with the aid of NMR technique. The results showed that the step-wise depressurization method could effectively prevent the ice generation and/or secondary hydrate formation phenomenon during NGH exploitation. When the pore pressure reached the hydrate dissociation surging point, the CH4 hydrate dissociation rate would enter a surge stage. Then, the hydrate dissociation rate of simple step-wise depressurization method kept steady for a relatively long time, while the hydrate dissociation rate of combined method showed a pressure-sensitive feature. And the optimum final CH4 recovery ratio of 92.85% was obtained in Case 7 with the finer step-wise depressurization combined method. The comparison between measured gas effective permeability and prediction value indicated that the accuracy of current widely used permeability prediction models was limited due to lacking consideration of wettability evolutions of hydrate bearing sediments. In this study, a novel NMR-based approach for quantitatively characterizing the overall wettability evolution of hydrate bearing sediments was proposed. It was found out that the hydrophilicity of hydrate bearing mixing system increased with the decrease of hydrate saturation during the dissociation process. And a modified Tokyo model coupled with wettability term was proposed to increase the normalized permeability prediction accuracy by 9.75%, which indicated the flow seepage behavior was governed by the coupling effect of hydrate pore habits and wettability of hydrate bearing sediments.

Pore-scale flow simulation on the permeability in hydrate-bearing sediments

Gas hydrate exploitation involves complex physical and chemical processes. Formation and dissociation of hydrate break the pressure-temperature phase equilibrium and cause change of pore-structure. The permeability of hydrate-bearing sediments plays a significant role for the fluid flow. The permeability change depends on the evolution of pore habit which is affected by hydrate phase behavior. Quantitative analyzing the relationship between pore structure and permeability still keeps a challenging work. To study the microscopic mechanism of hydrate effect on flow characteristic, the pore-scale simulation was carried out considering hydrate morphology and distribution. Five kinds of hydrate occurrence patterns including grain-coating, pore-filling, throat-filling, grain-cementing, and load-bearing in hydrate-bearing sediments are established. The permeability of hydrate-bearing sediments was obtained by solving the Navier-Stokes equation from the direct numerical simulation (DNS). The effect of factors related to the skeleton grain and hydrate distribution on permeability variation was analyzed. The results indicate that the permeability of hydrate-bearing porous media always changes with the variation of hydrate saturation. In most instance, the permeability reduction curve obeys exponential distribution. Permeability decreases as the hydrate saturation increases. The permeability variation is highly sensitive to the grain size, porosity, pore-throat size, hydrate occurrence pattern, and hydrate distribution morphology. From the perspective of the pore-scale, the formation and growth of hydrate cause the reconstruction and complication of pore structure in porous media. The permeability reduction caused by the hydrate formation is pronounced in the porous media with a larger initial pore-throat size. Considering different hydrate occurrence patterns and distribution morphology, the curve shape of the normalized permeability shows different trend. This new workflow has a potential application for analyzing the microscopic flow mechanism of hydrate-bearing sediments.

Permeability Analysis of Hydrate-Bearing Sediments during the Hydrate Formation Process

Permeability is a critical factor for controlling water and gas flow and influencing hydrate growth and distribution in hydrate-bearing sediments (HBS). In this study, we used in situ microfocus X-ray computed tomography to obtain three-dimensional information on microstructures of xenon hydrates in sedimentary matrices (Fujian sand) under excess-gas conditions. It was found that the hydrates in the specimen develop cementing and grain-coating pore habits, and the predominant hydrate pore habit is grain coating. We combined a pore network model simulation to analyze the effect of hydrate formation on the permeability in HBS. The results indicate that the water relative permeability decreases during the hydrate formation process. Gas relative permeability decreases at high water saturation (>0.20). The reason is that hydrate formation causes a decline in the pore space size and connectivity, and then, the gas and water flow are inhibited. Moreover, hydrate formation in HBS has an impact on its permeability anisotropy. Effective permeability decreases in x, y, and z directions with hydrate formation. The predominant hydrate pore habit in the pore space has a small influence on the evolution of the preferential flow direction of the water and gas. Predicting the dynamic permeability of the hydrate formation process would improve pore-scale water and gas flow analysis in HBS.

FRACTAL ANALYSES OF THE SHAPE FACTOR IN KOZENY–CARMAN EQUATION FOR HYDRAULIC PERMEABILITY IN HYDRATE-BEARING SEDIMENTS

Hydraulic permeability in hydrate-bearing sediments largely controlling the rate of gas production from hydrate deposits is widely estimated by various modified models based on the Kozeny–Carman equation. However, the effect of hydrate saturation on the shape factor in the Kozeny–Carman equation is not fully understood. In this study, a fractal model of the shape factor is theoretically derived to show physical relations between the shape factor and fractal parameters of fluid occupied pores. Then, verification of the model is performed, and effects of intrinsic porosity, intrinsic pore area and tortuosity fractal dimensions on the hydrate saturation dependent shape factor are analyzed. Finally, how weighted specific surface areas evolve with hydrate saturation is theoretically discussed, and results are further applied to modify the Kozeny–Carman equation. It is shown that the fractal model has a good performance on the shape factor estimation, and value of the shape factor is largely controlled by porosity, pore area and tortuosity fractal dimensions of fluid occupied pores in hydrate-bearing sediments. Multiplying product of the shape factor, the squared specific surface area, and the squared hydraulic tortuosity linearly increases with increasing hydrate saturation in general for the pore-filling hydrate pore habit but linearly decreases due to the presence of grain-coating gas hydrates. This linear relation permits a feasible modification of the Kozeny–Carman equation for hydrate-bearing sediments, and values of the model parameter are suggested for pore-filling and grain-coating hydrate pore habits.