Micro-CT Three-Dimensional Digital Modeling and Seepage Simulation of Coal under Liquid Nitrogen Fracturing.

Area of Science:

• Geology
• Petroleum Engineering
• Materials Science

Background:

• Coal's pore-fracture structure dictates its microstructural evolution and fluid transport properties.
• Understanding seepage characteristics is vital for coalbed methane (CBM) extraction efficiency.
• Low-rank bituminous coals present unique challenges due to their complex pore networks.

Purpose of the Study:

• To quantitatively analyze the 3D pore and fracture structure of coal before and after cryogenic liquid nitrogen fracturing.
• To evaluate the impact of fracturing on porosity, tortuosity, connectivity, and spatial distribution of pores and fractures.
• To establish relationships between pore structure parameters and permeability for improved CBM extraction.

Main Methods:

• Computed Tomography (CT) scanning for 3D imaging of pore-fracture networks.
• Mercury Injection Porosimetry (MIP) for pore size distribution analysis.
• Pore Network Modeling (PNM) to simulate fluid flow and calculate absolute permeability.

Main Results:

• Fracturing increased porosity from 6.32% to 10.39% and pore connectivity from 0.42 to 0.75.
• Fractured coal exhibited lower tortuosity, increased connectivity pore area and volume, and a more complex pore structure.
• Pore-throat parameters and coordination number in fractured coal followed a log-normal distribution, with coordination number increasing by 1.81.
• Permeability showed anisotropy and strong correlations with connectivity, coordination number, tortuosity, and fractal dimension, with coordination number being most influential.

Conclusions:

• Cryogenic liquid nitrogen fracturing significantly improves coal's pore structure, enhancing connectivity and fluid transport.
• The study provides a quantitative basis for optimizing fracturing techniques in coal seams.
• Findings are critical for advancing coalbed methane extraction and unconventional natural gas development.