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The gas-water two-phase seepage process is complex during the depressurization process of natural gas hydrate in a clayey silt reservoir in the South China Sea, the transport mechanism of which has not been clarified as it is affected by the pore structure. In this study, we select six clayey silt samples formed after the dissociation of natural gas hydrate in the South China Sea, employing CT scanning technology to observe the pore structure of clayey silt porous media directly. The original CT scanning images are further processed to get the binarized images of the samples, which can be used for simulation of the porosity and absolute permeability. Based on the fractal geometry theory, pore structures of the samples are quantitatively characterized from the aspect of pore distribution, heterogeneity, and anisotropy (represented by three main fractal geometric parameters: fractal dimension, lacunarity, and succolarity, respectively). As a comparison, the binarized CT images of two conventional sandstone cores are simulated with the same parameters. The results show that the correlation between porosity and permeability of the hydrate samples is poor, while there is a strong correlation among the succolarity and the permeability. Fractal dimension (represents complexity) of clayey silt samples is higher compared with conventional sandstone cores. Lacunarity explains the difference in permeability among samples from the perspective of pore throat diameter and connectivity. Succolarity indicates the extent to which the fluid in the pore is permeable, which can be used to characterize the anisotropy of pore structures. Therefore, these three fractal parameters clarify the relationship between the microstructure and macroscopic physical properties of clayey silt porous media. [ABSTRACT FROM AUTHOR]

Fractal theory has been widely used in petrophysical properties of porous rocks over several decades and determination of fractal dimensions is always the focus of researches and applications by means of fractal-based methods. In this work, a new method for calculating pore space fractal dimension and tortuosity fractal dimension of porous media is derived based on fractal capillary model assumption. The presented work establishes relationship between fractal dimensions and pore size distribution, which can be directly used to calculate the fractal dimensions. The published pore size distribution data for eight sandstone samples are used to calculate the fractal dimensions and simultaneously compared with prediction results from analytical expression. In addition, the proposed fractal dimension method is also tested through Micro-CT images of three sandstone cores, and are compared with fractal dimensions by box-counting algorithm. The test results also prove a self-similar fractal range in sandstone when excluding smaller pores. [ABSTRACT FROM AUTHOR]

Spontaneous imbibition widely occurs during the industrial development of tight reservoirs. Clarifying the spontaneous imbibition mechanisms and the key influencing factors contributes to the recovery of such reservoirs. In this study, basic tests of petrophysical properties and spontaneous imbibition experiments are conducted on 11 tight sandstone cores sampled from Dongying Sag, Bohai Bay Basin, China. The mass change and distribution of imbibed water are monitored by balance and nuclear magnetic resonance during the imbibition experiments. The petrophysical properties and fractal parameters are implemented to investigate the factors influencing imbibition in tight sandstone. The imbibition curves of 11 samples display a relatively similar shape but have differences in the final mass, the time required to reach the final mass and the slopes of the early stage. The effect of pore characteristics on imbibition process is explicated by nuclear magnetic resonance results. In the double logarithmic diagram, the imbibition curves can be divided into two regions. The main influencing factors of the final dimensionless imbibition mass are porosity, fractal dimension, and succolarity, indicating that the amount, complex characteristic, and connectivity of pores are critical for imbibition. However, no obvious relationship could be established between the permeability, lacunarity, and final dimensionless imbibition mass. The results assist with the clarification of the imbibition effect and its influencing factors during the industrial development of tight sandstone reservoirs. • Petrophysical properties and imbibition are tested on 11 tight sandstones. • Migration and distribution of imbibed water during imbibition are monitored. • Influencing factors of imbibition are analyzed from different perspectives. [ABSTRACT FROM AUTHOR]

The complex and heterogeneous microstructure of underground porous media usually results from geological processes such as deposition, compaction, cementation and fluid-mineral interactions. Variations in these microstructural characteristics largely determine the differences in the transport properties of porous media. The fractal geometry theory has been successfully applied to characterize the microstructure and transport properties of underground reservoirs. In this study, two-dimensional porous media with differently heterogeneous structures are reconstructed by utilizing the quartet structure generation set method. Fractal dimension and lacunarity are implemented to quantify the complex microstructure and analyze the relationships among these parameters. Under specific porosity, the fractal dimension increases with the reduction of lacunarity according to a power law relationship. Meanwhile, permeability has a perfect power law relationship with porosity under the same heterogeneity. The stronger heterogeneity increases the degree of pore aggregation, resulting in increased pore radius for fluid flow. For porous media with the same porosity, permeability has a significant positive power law relationship with lacunarity and a negative relationship with fractal dimension. The results demonstrate that fractal parameters are helpful to fully understand the influence of complex microstructure and chemical reactions on the macroscopic physical properties of porous media. • Two-dimensional porous media with differently complex and heterogeneous pore structures are reconstructed. • Fractal parameters are implemented to quantify the microstructure and analyze the relationships among these parameters. • The relationships between permeability and pore structural parameters are established. [ABSTRACT FROM AUTHOR]

• Three fractal parameters (dimension, lacunarity and succolarity) were employed to characterize the microstructure of reservoir rocks. • Twelve 3D CT images of sandstone reservoir rocks with different permeabilities were used to estimate the three fractal parameters. • The fractal parameters, especially succolarity, identified differences in core micro-structure as related to differences in permeability. An explicit relationship between seepage properties and microscopic structure of porous media is being sought for the development of oil and gas resources. An effective method to accurately characterize and quantify the microscopic structure of porous media is a key issue. Fractal geometry can take advantage of several physically-based parameters to analyze microstructures of reservoir rocks. In this study, three fractal structural parameters, fractal dimension, lacunarity and succolarity, were employed to characterize scale-invariant complexity, heterogeneity, and anisotropy of rock microstructures, respectively. Twelve three-dimensional digital cores of sandstone reservoir rocks were used to evaluate permeability in terms of fractal dimension, lacunarity and succolarity. The parameters were utilized to quantitatively characterize differences in core micro-structure and predict their effects on permeability. Due to the confounding influence of porosity, the fractal dimension was not an accurate predictor of the variation in permeability on its own. Instead, the results reveal that lacunarity and succolarity were better able to predict differences in structure and permeability. Succolarity, in particular, showed an exponential relationship with permeability, yielding a coefficient of determination of 0.940. Using a combination of fractal structural parameters in the place of pore-size distribution, can provide a better explanation of the relationship between fluid flow, and the heterogeneous structure and anisotropic physical properties of reservoir rocks. [ABSTRACT FROM AUTHOR]