Pore Structure Fractal Characterization and Permeability Simulation of Natural Gas Hydrate Reservoir Based on CT Images

Abstract

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.

Influence of Particle Size Distribution on the Physical Characteristics of Pore-Filling Hydrate-Bearing Sediment

“…In these models, the yellow area represents the sand, while the gray area represents the hydrate. In addition, it could be found that the radius of hydrate particles in pore space would increase gradually as the hydrate saturation increases, and some hydrate particles that do not contact with the sand particles (pore-filling type) will gradually contact with the sand particles (load-bearing type), which is consistent with previous studies [28,29]. Specifically, hydrate saturation calculation was as follows:…”

“…Characteristic Evolution of Pore Space. The pore space could impact the reservoir capacity significantly [29,30], and the PNM is an effective tool to evaluate pore space evolution. In this study, PNM was utilized to estimate the pore and throat characteristics in the porous media under different particle sizes.…”

Advances in Characterizing Gas Hydrate Formation in Sediments with NMR Transverse Relaxation Time

“…CT technology distinguishes materials by their different electron densities and attenuation coefficients [107]. The CT images the habits of hydrate pore with a resolution of tens of microns to sub microns in Figure 8a,b [103,107,108,[110][111][112]. The scanning time is inversely proportional to the resolution and can reach dozens of hours at a resolution of 1 micron [107,108,110].…”

“…According to the 3D image, the distributions of the pore size and saturation can be calculated. The permeability can be predicted using Lattice Boltzmann modeling, pore network model, or other predictive permeability models [103,110,117]. However, in CT measurement, the attenuation coefficients of methane hydrates and water (due to their minuscule density difference) are too close to identify methane hydrate segmentation.…”

“…The lithology of the hydrate reservoir is clayey silt, and the main minerals are quartz, feldspar, carbonate, and clay [4]. The mean grain size of sediments is tens of microns [4,103]. The pore diameter of hydrate-bearing sediments in the Shenhu area of the South China Sea ranges from 500 nm to 20 µm.…”