Abstract
The CO2 phase state changes during the long-term interaction with coal seams. A proper understanding of the influence of different CO2 phase states in water-bearing coal seams is of great significance in evaluating the leakage risk during CO2 sequestration. The evolution of fluid flow pathways in water-bearing coal subjected to the treatment of different CO2 phase states was investigated by using X-ray computed tomography (CT) technology, fractal dimension, and three-dimensional (3D) pore-scale flow modeling. The results show that the supercritical CO2 (ScCO2) treatment reduced the coal heterogeneity by 12.69% and increased the absolute permeability by 58.75%. Conversely, only 0.71% of coal heterogeneity reduction occurred after subcritical CO2 (SubCO2) treatment, increasing the absolute permeability by 24.91%. The reduction in the tortuosity fractal dimension after ScCO2 treatment was 2.69% larger than that of SubCO2 treatment, indicating that ScCO2 treatment was more favorable for improving the transport capacity of flow pathways. The pressure field distributions in the pore network model (PNM) were determined by coal heterogeneity and influenced by the number of flow pathways. The effect of ScCO2 treatment on the size, quantity, and location of preferential flow pathways in coal is more significant than that of SubCO2 treatment. Moreover, the proportion of preferential flow pathways in ScCO2-treated coal was less than 50%, much lower than that of the throats in PNMs. In contrast, the proportion of preferential flow pathways in SubCO2-treated coal was approximately 30% higher than that in ScCO2-treated coal. The distribution of preferential flow pathways also indicated that not all pores with larger radii participated in preferential flow. Furthermore, more than 50% of the pathways in the ScCO2-treated coal did not contribute to fluid flow, and could affect the stability of the coal seam.
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Abbreviations
- ROI:
-
Region of interest
- N(s) :
-
Smallest number of spheres or cube
- A.D.:
-
Average deviation
- D f :
-
Fractal dimension of pore area
- D M :
-
Fractal dimension of the point set
- D τ :
-
Fractal dimension of tortuosity
- F :
-
Flow rate frequency
- g ij :
-
Conductance of the throat
- L 0 :
-
Straight length of the capillary
- l ij :
-
Cylindrical pipes of length
- L τ :
-
True streamline length
- M :
-
Point set
- n :
-
Number of slices
- N :
-
Number frequency
- P:
-
Pressure of the fluid
- q ij :
-
Flow rate between pore i and pore j
- r ij :
-
Cylindrical pipes of radius
- S :
-
Side length
- t :
-
Time
- n :
-
Normal unit vector
- β 0 :
-
Number of isolated pore-fractures
- β 1 :
-
Number of connected pore-fractures
- β 2 :
-
Number of closed pore-fractures
- λ:
-
Diameter of curved capillary
- λav :
-
Average diameter of the capillaries
- λmax :
-
Largest diameter
- λmin :
-
Smallest diameter
- τ :
-
Tortuosity
- τav :
-
Average tortuosity of the capillaries
- ϕ :
-
Porosity
- \(\overrightarrow{\nabla \cdot }\) :
-
Divergence operator
- \(\overrightarrow{\nabla}\) :
-
Gradient operator
- \(\overrightarrow{V}\) :
-
Velocity of the fluid
- \(\mu\) :
-
Fluid dynamic viscosity
- \({\nabla }^{2}\) :
-
Laplacian operator
- \({\phi }_{A}\) :
-
Area porosity of each slice
- \(\overline{{\phi }_{A}}\) :
-
Average area porosity
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This study was financially supported by the National Natural Science Foundation of China (Grant No. 51674047 & Grant No. 51911530152 & Grant No. 52204034)
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Luo, P., Zhang, Z., Zhang, L. et al. Influence of different CO2 phase states on fluid flow pathways in coal: insights from image reconstruction and fractal study. Bull Eng Geol Environ 82, 266 (2023). https://doi.org/10.1007/s10064-023-03322-0
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DOI: https://doi.org/10.1007/s10064-023-03322-0