Pore and permeability changes in coal induced by true triaxial supercritical carbon dioxide fracturing based on low-field nuclear magnetic resonance

Supercritical carbon dioxide (ScCO2) fracturing is a green, clean, waterless extraction technique that has gained widespread attention. Coal pore properties, such as porosity, pore size distribution, connectivity, and permeability, are critical for fracturing and efficient coalbed methane production. This study examines the effects of ScCO2 fracturing on coal reservoir pore modification by conducting true triaxial ScCO2 fracturing experiments on high-rank coal samples under various stresses and injection rates. Low field nuclear magnetic resonance technique was used to compare and analyse the pore permeability characteristics of the samples before and after ScCO2 fracturing. The research discusses the influence of stress and ScCO2 fracturing fluid injection rate on coal pore modification and its controlling mechanisms. The results show a significant impact on coal pore modification, with a 59.85 % increase in porosity, 60 % increase in pore volume, 56 % increase in pore throat volume, and 47.5 % increase in permeability. Under the same injection rate and fixed temperature (40 °C), higher stress differences (8 MPa) benefit large pore modification and connectivity, while lower stress differences (4 MPa) are more favourable for micropore and transition pore modification and connectivity. These findings contribute to a deeper understanding of the microscopic mechanisms of ScCO2 fracturing in modifying coal pores. •The changes of porosity and permeability caused by fracturing are analyzed.•The spatial analysis of the fracturing samples was conducted at different locations.•The two-dimensional image vividly represents the changes before and after fracturing.•Larger in-situ stress difference is conducive to the reconstruction of macropore.•The lower injection rate is conducive to increasing the connectivity between micropore.