Damage Effects and Fractal Characteristics of Coal Pore Structure during Liquid CO2 Injection into a Coal Bed for E-CBM

Abstract

Pore structure has a significant influence on coal-bed methane (CBM) enhancement. Injecting liquid CO 2 into coal seams is an effective way to increase CBM recovery. However, there has been insufficient research regarding the damage effects and fractal characteristics of pore structure at low temperature induced by injecting liquid CO 2 into coal samples. Therefore, the methods of low-pressure nitrogen adsorption-desorption (LP-N 2 -Ad) and mercury intrusion porosimetry (MIP) were used to investigate the damage effects and fractal characteristics of pore structure with full aperture as the specimens were frozen by liquid CO 2 . The adsorption isotherms revealed that the tested coal samples belonged to type B, indicating that they contained many bottle and narrow-slit shaped pores. The average pore diameter (APD; average growth rate of 18.20%), specific surface area (SSA; average growth rate of 7.38%), and total pore volume (TPV; average growth rate of 18.26%) increased after the specimens were infiltrated by liquid CO 2 , which indicated the generation of new pores and the transformation of original pores. Fractal dimensions D 1 (average of 2.58) and D 2 (average of 2.90) of treated coal samples were both larger the raw coal (D 1 , average of 2.55 and D 2 , average of 2.87), which indicated that the treated specimens had more rough pore surfaces and complex internal pore structures than the raw coal samples. The seepage capacity was increased because D 4 (average of 2.91) of the treated specimens was also higher than the raw specimens (D 4 , average of 2.86). The grey relational coefficient between the fractal dimension and pore structure parameters demonstrated that the SSA, APD, and porosity positively influenced the fractal features of the coal samples, whereas the TPV and permeability exerted negative influences.

Effect of Division Methods of the Adsorption Isotherm on the Fractal Dimension of Clay Minerals Calculated Based on the Frenkel–Halsey–Hill Model

“…For those samples with two linear stages, in III 1 stage, after the judgment of eq , eq was applied to calculate the fractal dimension of the KI13 sample, and eq was applied for M, K, and I samples. However eq also was applied for KI13, M, K, and I in the III 2 stage based on previous studies. ,, The calculation results of D, the slopes, and the R 2 of fitting lines in Figure are shown in Table .…”

“…Based on the observation of the FHH model fitting lines on the adsorption isotherms in different samples, ,− it is found that there is no uniform method for dividing the adsorption isotherms when this model was applied. Moreover, the division method has great effect on the calculation result of the fractal dimension.…”

“…According to the selected cutoff point, the existing methods can be divided into three categories. In method I, the starting point of the hysteresis loop was taken as the cutoff point and the turning point of the adsorption mechanism from multilayer adsorption to capillary condensation. − This method is the most commonly used method in computation. However, the fitting lines of the two stages of some samples are collinear in these results (Figure a,b), , which means that the adsorption mechanism may not have changed.…”

Influence of supercritical, liquid, and gaseous CO2 on fracture behavior in sandstone

“…In other words, when the rock breaks, the required external load decreases. Yin Hong et al, 31 Perera et al, 34 and Ma Li et al 66 have shown that the CO 2 adsorption of rocks can significantly reduce the surface energy of rocks, especially for ScCO 2 . A strong adsorption capacity has the most significant effect on reducing rock surface energy and has the most prominent rock strength deterioration.…”

Experimental Study on Explosion Pressure and Rock Breaking Characteristics under Liquid Carbon Dioxide Blasting

“…Kang et al [21] analyzed the phase transition process of liquid carbon dioxide and calculated the total energy released in this process. Ma et al [22] analyzed the fractal characteristics and damage effects of the pore structure of specimens frozen by liquid carbon dioxide.…”