Investigation of temperature effects from LCO2 with different cycle parameters on the coal pore variation based on infrared thermal imagery and low-field nuclear magnetic resonance

Enhanced coalbed methane (ECBM) achieved by injecting liquid carbon dioxide (LCO2) has been proposed and applied in industrial production for decades and has been demonstrated to be an applicable method to boost CBM production. Most of the studies have concentrated on the gas bursting and flooding effect and have rarely focused on the accompanying "freeze-thaw" phenomenon, and the temperature effect of cyclic LCO2 injection on the pore variation of different coals has been partly investigated. In this paper, the influence of cycle parameters, such as cycle number and cycle time, on the pore variation was studied. Infrared thermal imagery (ITI) and low-field nuclear magnetic resonance (NMR) were used to measure the temperature and pore size distribution (PSD) change, respectively. The results show the following: (1) The gas pressure displayed square cyclicity with different cycle time, the temperature of gasified CO2 was almost 248.15 K, and the end and lateral surface temperatures of a core were in the range from 259.35 to 261.85 K, which could cause the water within the pores to freeze with a 9% volume increase, and the fracturing formula was deduced; (2) The relaxation time spectra obtained by different cycle parameters expressed changeable PSD of cores with increasing cycle parameters, and the magnified proportion of bulk water and capillary water, as well the diminished proportion of adsorbed water, all indicated that the increased number of macropores and mesopores formed a larger free volume; (3) The increased total porosity phi(t) and the decreased T-2cutoff of six cores with the increasing cycle parameters meant that the larger cycle number could enhance the porosity due to amount of damage accumulation, and the larger cycle time might make the water freeze completely with larger ice swelling stress; (4) There is a polynomial fitting between relative increase ratio R phi and cycle time, and the fitting coefficients were all higher than 0.99, and the larger the cycle time was, the greater the R phi((e/t)) increment and R phi((r/t)) decrement were. The interval increase ratio I phi(e) was positively correlated to cycle time without obvious increase behavior; however, the I phi(r) variation expressed that the greater the cycle number was, the lesser the I phi(r) with the increasing cycle time was, which indicates that the increasing cycle parameters might help the proportion of connected pores to increase and provide more pathways for permeable fluid; (5) The NMR permeability k(SDR) of a core increased as the cycle number increased, and the longer cycle time was superior in terms of permeability enhancement.