Pore evolution of coals affected by cyclical liquid CO2 fracturing based on the low-field nuclear magnetic resonances

The low porosity and permeability of coals have been the main bottleneck restricting the efficient gas extraction.Nowadays, the pore structure of coal reservoirs is mostly reformed by the fracturing and anti-reflection technology to improve permeability.The liquid CO2 cyclical fracturing can synergistically destroy the coals by the coupling effect of thermal cycling, phase change fracturing and fatigue damage, and the low-field nuclear magnetic resonance technology can be used to achieve a quantitative characterization of pore-crack structure of coals.However, few studies focus on the evolution law of pore geometric characteristics of liquid CO2 affected coals based on the nuclear magnetic resonances.Based on the liquid CO2 cyclical test platform dependently developed, the lignite specimens are affected by multiple liquid CO2 impacts, and the pore change are recorded by nuclear magnetic relaxation technology.The fractal geometry theory is adopted to explore the relationships among the seepage pore fractal dimension Ds, the effective pore fractal dimension De, the T2 cutoff value, porosity, and permeability.The relationships among the crack morphology difference, the matrix heterogeneity and original pore structure of affected coals by liquid CO2 are statistically analyzed.The T2 spectra obtained by the “saturation-centrifuge” method indicates that total porosity φt, effective porosity φe, nuclear magnetic permeability kSDR-d and its increase rate ΔkSDR-d all increase.Geometry fractal calculation model is built based on the T2 spectrum, demonstrating that the seepage pores and the effective connected pores have better fractal characteristics and the adsorbed pores do not have fractal characteristics.Ds and T2 cutoff values have an exponential fitting relationship with “rapid increase-slow increase”, and have negative relationships with φt, φe, kSDR-d and ΔkSDR-d, respectively.De positively correlates withφe and ΔkSDR-d, respectively.These reveal the fact that the liquid CO2 cyclical impact could enhance multi-scale pore expansion and crack extension, and the crack penetration and permeability of coals substantially increase.This method could provide some basic supports for the quantitative evaluation of pore structure under the effect of liquid CO2 cyclical fracturing in the field application.