Numerical Simulation of Matrix Swelling and Its Effects on Fracture Structure and Permeability for a High-Rank Coal Based on X-ray Micro-CT Image Processing Techniques

Abstract

The adsorption gas of coal matrix will cause its swelling, change the fracture structure, and further affect the permeability of the coal seam. In this study, CT image processing technology was used to numerically simulate the swelling of different extents (0.05%, 0.10%, 0.15%, 0.20%, and 0.25%). At the same time, the evolutions of geometric parameters, connectivity, and fractal dimension of fractures under different swelling extents were quantified. The results show that with the increase of the swelling extents, the average length, width, and aperture of fractures decreased by 18.69%, 24.14%, and 52.24%, respectively. The connected porosity of fractures decreased by 44.83%, indicating fractures are closed and the connectivity becomes worse. It is noteworthy that when the swelling extent is less than 0.1%, the average aperture and connected porosity of fractures are significantly reduced by 32.84% and 31.03%, respectively. When it is greater than 0.15%, and the average aperture and width of fractures are clearly reduced by 49.25% and 23.28%, respectively. The fractal dimension increased from 2.132 to 2.211, indicating that the fracture structure became complex. Additionally, using Darcy’s law to determine the change in absolute permeability during swelling, the results show that there was a significant negative exponential correlation between permeability and swelling extents. The decrease of aperture and connected porosity are the main factors of poor permeability.

Correlation Evaluation and Schematic Analysis of Influencing Factors Affecting Pore and Fracture Connectivity on the Microscale and Their Application Discussion in Coal Reservoir Based on X-ray CT Data

“…Absolute permeability can be defined as the measure of the ability of a porous material to transmit a single phase fluid. , The Stokes equations can be solved for numerically estimating the absolute permeability, , as can be seen in formula : true{ lefttrue ∇ · true⃗ V⃗ = 0 μ ∇ 2 V⃗ − ∇⃗ P = 0 Where → ∇ · is the divergence operator; → ∇ is the gradient operator; → V is the velocity of the fluid in the fluid phase of the material; μ is still the dynamic viscosity of the flowing fluid; ∇ 2 is the Laplacian operator; and P is the pressure of the fluid in the fluid phase of the material.…”

“…Absolute permeability can be defined as the measure of the ability of a porous material to transmit a single phase fluid. 31,32 The Stokes equations can be solved for numerically estimating the absolute permeability, 33,34 as can be seen in formula 4:…”

Review on Applications of X-ray Computed Tomography for Coal Characterization: Recent Progress and Perspectives

“…In the 3D reconstruction of the coal microstructure, the specific grayscale ranges of pore fractures, minerals, and matrix normally determined by threshold segmentation. , As an important basis for 3D reconstruction, the accuracy of threshold determines the accuracy of the final 3D reconstruction model, and the existence of noise in the image makes deviation from the real image inevitable . Therefore, the CT images usually require further processing, including adjusting the image contrast and filtering the scatter dots noise prior to segmentation. − Using the preprocessed three-dimensional (3D) image, the threshold segmentation is calibrated along with measured porosity (MIP or other methods), and then used to obtain the 3D coal microstructure (see Figure ). ,, …”

“…Adsorption time, adsorption pressure, and the microstructure affect the swelling deformation of the coal matrix after gas adsorption, and the changes in the porosity and permeability of coal after adsorption are closely related to the gas pressure in the pores and fractures and the degree of coal matrix swelling deformation. By evaluating the internal microstructure of coal under different adsorption pressures using CT and simulating coal swelling deformation using the Avizo software, the results showed that, as the adsorption pressure of gas increases, the thickness of typical fractures in the coal decreases significantly (see Figure ), causing a negative exponential decrease in porosity and permeability. , The preswelling stage of the coal matrix shows a significant decrease in fracture length, followed by a substantial decrease in fracture width …”

“…106,107 The preswelling stage of the coal matrix shows a significant decrease in fracture length, followed by a substantial decrease in fracture width. 41 In addition to increasing the adsorption amount and enhancing the matrix swelling deformation, the gas pressure promotes the porosity and permeability of coal. For the loaded coal samples, the reduction in effective stress caused by the entry of high-pressure gas into the coal fractures immediately increases the fracture size.…”