Damage Effects and Fractal Characteristics of Coal Pore Structure during Liquid CO2 Injection into a Coal Bed for E-CBM
Abstract
:1. Introduction
2. Coal Samples and Methods
2.1. Coal Samples Re-Preparation
- (1)
- Three specimens were infiltrated in LqCO2 at −50 °C for 4 h with self-developed equipment. A previous study indicated that the cooling radius of LqCO2 injected into coal seam was approximately 10 m, meanwhile the coal around the injection hole was fully infiltrated after 4 h of LqCO2 injection [25,26].
- (2)
- After the infiltration process, raw and treated coal samples were added into a vacuum oven at 65 °C for 12 h until the mass changes in the initial and treated coal samples did not exceed 0.2% [5].
- (3)
- All the coal samples were desorbed in a vacuum chamber for 48 h to remove the mixed gases adsorbed on the surface of the coal samples.
2.2. Fractal Dimension from LP-N2-Ad Isotherms
2.3. Fractal Dimension from MIP
2.4. Grey System Theory
3. Results and Discussion
3.1. Evolution and Fractal Characteristics of Adsorbed Pores
3.1.1. LP-N2-Ad Isotherms and Pore Structure Parameters Analysis of Adsorbed Pores
3.1.2. Fractal Characteristics of the Adsorbed Pores from Raw and LqCO2-Treated Coal Samples
3.2. Variations and Fractal Characteristics of Seepage Pores
3.2.1. MIP Curves
3.2.2. Pore Structure Parameters Analysis of Seepage Pores
3.2.3. Fractal Features of Seepage Pores between the Initial and LqCO2-Treated Coal Samples
3.3. Grey Relational Application and Discussion
3.3.1. Calculated Degrees of Correlation
3.3.2. Characterization of the Fractal Dimension and Coal Pore Structure Parameters by the Grey Correlation Degree
4. Conclusions
- (1)
- The adsorption isotherms of three coal specimens were of type B, which illustrates that the coal samples contained numerous cylindrical shaped-pores with one closed side and slit-shaped and bottle-shaped pores. From the LP-N2-Ad and MIP tests, the APD (average growth rate of 18.20%), SSA (average growth rate of 7.38%), and TPV (average growth rate of 18.26%) were higher after the coal specimens were infiltrated by liquid CO2. This is because of the large number of new pores generated and plenty of micropores and transition pores transferred into mesopores and macropores. Therefore, the adsorption ability was lower while the seepage capacity was higher, which is suitable for CBM recovery.
- (2)
- Fractal dimensions D1 (average of 2.58), D2 (average of 2.90), and D4 (average of 2.91) exhibited the same tendencies for the treated coal samples, which were typically higher than those of the original specimens (D1, average of 2.55, D2, average of 2.87, and D4, average of 2.86), indicating that the coal surfaces are rougher and the internal pore structures are more complex after the coal samples were treated by LCO2.
- (3)
- The grey relational theory was applied to analyze the relationship between fractal dimension and coal pore structure parameters. The correlation degree was higher than , indicating that the fractal features of the treated coal specimens are more evident. The degree of correlation between the fractal dimension and pore structure parameters show that the SSA of the raw and treated coal samples was largest, followed by APD, porosity, TPV, and permeability. The SSA, APD, and porosity positively influence the fractal characteristics of coal samples, but TPV and permeability exert negative influences.
Author Contributions
Funding
Conflicts of Interest
References
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Sample No. | R0 (%) | Coal Macerals (%) | Proximate and Ultimate (wt %) | Coal Species | |||||
---|---|---|---|---|---|---|---|---|---|
V | I | E | M,ad | A,d | V,daf | FC,ad | |||
MHC | 2.20 | 62.06 | 26.90 | 1.70 | 1.53 | 21.95 | 12.50 | 65.14 | Meager coal |
MSC | 1.90 | 63.38 | 29.15 | 2.10 | 1.67 | 22.47 | 12.46 | 65.43 | Meager coal |
1/3CC | 0.99 | 52.60 | 31.60 | 15.80 | 1.67 | 25.71 | 38.87 | 33.75 | 1/3 cocking coal |
Sample No. | SBET,Ad (m2/g) | Smi+tra (m2/g) | VBJH,Ad (cm3/g) | Vmi+tra (cm3/g) | ||||||
---|---|---|---|---|---|---|---|---|---|---|
Smi | Proportion/% | Stra | Proportion/% | Vmi | Proportion/% | Vtra | Proportion/% | |||
MHC untreated | 0.96 | 0.32 | 32.7 | 0.66 | 67.0 | 0.0035 | 0.00039 | 11.0 | 0.0032 | 89.0 |
MHC treated | 1.26 | 0.42 | 33.5 | 0.84 | 66.5 | 0.0042 | 0.00013 | 3.1 | 0.0040 | 96.9 |
MSC untreated | 0.81 | 0.36 | 49.1 | 0.41 | 50.9 | 0.0015 | 0.00038 | 25.0 | 0.0011 | 75.0 |
MSC treated | 0.92 | 0.40 | 43.8 | 0.51 | 56.2 | 0.0017 | 0.00024 | 13.8 | 0.0015 | 86.2 |
1/3CC untreated | 3.10 | 1.57 | 50.4 | 1.54 | 49.6 | 0.0077 | 0.00023 | 29.4 | 0.0055 | 70.6 |
1/3CC treated | 3.24 | 1.58 | 48.7 | 1.66 | 51.3 | 0.0088 | 0.00021 | 23.9 | 0.0067 | 76.1 |
Coal Sample | D = 3 + A | |||||
---|---|---|---|---|---|---|
Relative Pressure (P/P0): 0.0–0.5 | Relative Pressure (P/P0): 0.5–1.0 | |||||
A2 | D2 | R22 | A1 | D1 | R12 | |
MHC untreated | −0.090 | 2.90 | 0.792 | −0.545 | 2.45 | 0.995 |
MHC treated | −0.088 | 2.91 | 0.717 | −0.532 | 2.47 | 0.995 |
MSC untreated | −0.073 | 2.92 | 0.874 | −0.375 | 2.63 | 0.987 |
MSC treated | −0.069 | 2.94 | 0.835 | −0.351 | 2.65 | 0.970 |
1/3CC untreated | −0.171 | 2.82 | 0.950 | −0.435 | 2.56 | 0.991 |
1/3CC treated | −0.152 | 2.85 | 0.960 | −0.391 | 2.61 | 0.999 |
Sample No. | Φ (%) | K (mD) | APD (nm) | St (m2/g) | Sme+ma (cm2/g) | Vt (cm3/g) | Vme+ma (cm3/g) | ||
---|---|---|---|---|---|---|---|---|---|
Sme | Sma | Vme | Vma | ||||||
MHC-untreated | 5.47 | 19.6 | 15.1 | 10.14 | 0.044 | 0.013 | 0.038 | 0.0033 | 0.011 |
MHC-treated | 7.02 | 21.5 | 18.9 | 10.82 | 0.026 | 0.027 | 0.051 | 0.0028 | 0.028 |
MSC-untreated | 7.63 | 28.1 | 19.2 | 12.58 | 0.030 | 0.028 | 0.063 | 0.0041 | 0.018 |
MSC-treated | 8.59 | 30.2 | 22.9 | 13.16 | 0.012 | 0.035 | 0.072 | 0.0018 | 0.038 |
1/3 CC-untreated | 8.77 | 18.8 | 20.7 | 12.22 | 0.046 | 0.038 | 0.070 | 0.0051 | 0.016 |
1/3 CC-treated | 9.44 | 20.0 | 25.4 | 13.79 | 0.021 | 0.046 | 0.077 | 0.0035 | 0.040 |
Coal Samples | APD (100–1000 nm) | APD (1000–20,000 nm) | APD (>20,000 nm) | ||||||
---|---|---|---|---|---|---|---|---|---|
R32 | A3 | D3 | R42 | A4 | D4 | R52 | A5 | D5 | |
MHC untreated | 0.841 | −2.310 | 1.69 | 0.987 | −1.104 | 2.89 | 0.640 | −0.224 | 3.78 |
MHC treated | 0.857 | −2.510 | 1.49 | 0.976 | −1.030 | 2.97 | 0.849 | −0.326 | 3.67 |
MSC untreated | 0.857 | −2.002 | 1.99 | 0.935 | −1.129 | 2.87 | 0.550 | −0.158 | 3.84 |
MSC treated | 0.865 | −2.124 | 1.88 | 0.946 | −1.120 | 2.88 | 0.356 | −0.118 | 3.88 |
1/3CC untreated | 0.824 | −1.965 | 2.03 | 0.954 | −1.165 | 2.83 | 0.429 | −0.214 | 3.79 |
1/3CC treated | 0.793 | −2.315 | 1.68 | 0.878 | −1.106 | 2.89 | 0.669 | −0.171 | 3.83 |
Sample No. | for Raw Coal Samples | for LCO2 Frozen Treated Coal Samples | ||||||||
---|---|---|---|---|---|---|---|---|---|---|
MHC | 0.260 | 0.124 | 0.186 | 0.139 | 0.338 | 0.172 | 0.086 | 0.175 | 0.143 | 0.255 |
MSC | 0.046 | 0.267 | 0.046 | 0.079 | 0.106 | 0.035 | 0.235 | 0.034 | 0.017 | 0.055 |
1/3CC | 0.214 | 0.143 | 0.140 | 0.060 | 0.241 | 0.108 | 0.148 | 0.032 | 0.032 | 0.132 |
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Ma, L.; Wei, G.; Li, Z.; Wang, Q.; Wang, W. Damage Effects and Fractal Characteristics of Coal Pore Structure during Liquid CO2 Injection into a Coal Bed for E-CBM. Resources 2018, 7, 30. https://doi.org/10.3390/resources7020030
Ma L, Wei G, Li Z, Wang Q, Wang W. Damage Effects and Fractal Characteristics of Coal Pore Structure during Liquid CO2 Injection into a Coal Bed for E-CBM. Resources. 2018; 7(2):30. https://doi.org/10.3390/resources7020030
Chicago/Turabian StyleMa, Li, Gaoming Wei, Zhenbao Li, Qiuhong Wang, and Weifeng Wang. 2018. "Damage Effects and Fractal Characteristics of Coal Pore Structure during Liquid CO2 Injection into a Coal Bed for E-CBM" Resources 7, no. 2: 30. https://doi.org/10.3390/resources7020030
APA StyleMa, L., Wei, G., Li, Z., Wang, Q., & Wang, W. (2018). Damage Effects and Fractal Characteristics of Coal Pore Structure during Liquid CO2 Injection into a Coal Bed for E-CBM. Resources, 7(2), 30. https://doi.org/10.3390/resources7020030