Mitigation of Fracturing Fluid Leak-Off and Subsequent Formation Damage Caused by Coal Fine Invasion in Fractures: An Experimental Study
Abstract
:1. Introduction
2. Experimental Methodology
2.1. Sample Preparation
2.2. Experimental Apparatus
2.3. Experimental Conditions and Procedures
- (1)
- Preparation: After vacuuming and saturating the prepared coal sample with a 2% KCl solution for 48 h, load it into the core holder. Start pump B and set the confining pressure to 3 MPa. Load the prepared coal fine suspension into the intermediate container and continue stirring to ensure uniform dispersion in the 2% KCl solution.
- (2)
- Initial permeability measurement: Open valve B and start pump A. Set the pump A liquid injection rate to 1 mL/min of 2% KCl solution into the core holder. Monitor the pressure difference across the core holder in real time. Once the pressure difference stabilizes, measure the initial permeability of the coal sample. The permeability calculation follows Darcy’s law:
- (3)
- Temporary plugging experiment: Close valve B and pump A, then sequentially open valve A and the gas cylinder. Set the output pressure of the gas cylinder to 2 MPa and inject the liquid from the intermediate container into the core holder at constant pressure. Continuously monitor the leak-off of the fracturing fluid and calculate the temporary plugging rate of coal fines in the fracturing fluid:
- (4)
- Unplugging experiment: Close the gas cylinder and valve A in sequence, then open valve B and start pump A. Inject 2% KCl solution into the core holder at 1 mL/min. Monitor the pressure difference across the core holder in real time and collect the output liquid during the unplugging process. Calculate the damage rate of coal fines to the permeability of the coal sample after unplugging:
- (5)
- Post-experimental processing: Release the pressure from the experimental system, remove the coal sample, clean the coal sample and equipment, and restore the initial operational settings of the apparatus. Varying the coal fine parameters for Tests No. 1–7, repeat steps 1–5. Analyze the impact of coal fines of different concentrations and particle sizes in the fracturing fluid on the temporary plugging rate and the permeability damage rate.
3. Results and Discussion
3.1. Characteristics of Temporary Plugging of Coal Fines
3.2. Strength of Plugging and Mechanisms of Unplugging
3.3. Formation Damage Caused by Temporary Plugging with Coal Fines
4. Conclusions
- (1)
- Coal fines act as a natural “temporary plugging agent” to reduce fracturing fluid leak-off, and temporary plugging rates increase with higher concentrations and decrease with larger particle sizes.
- (2)
- The high temporary plugging effect of coal fines occurs through the combination of internal and external filter cakes. Increased coal fine concentration enhances plugging strength, while larger particle sizes reduce it. The permeability damage rate caused by temporary plugging coal fines in fractures initially increases and then decreases with increasing concentration and particle size. However, relatively lower damage rates occur under the conditions of smaller particle sizes and higher concentrations.
- (3)
- In balancing the temporary plugging rate and damage rate of coal fines on coal seam fractures, it is beneficial to maintain a lower concentration of coal fines in the fracturing fluid while ensuring a high temporary plugging rate. This approach helps mitigate the damage to the CBM production channel system during extraction. Simultaneously, during the hydraulic fracturing stage, temporary plugging by coal fines can compensate for fracturing fluid leak-off, thereby enhancing the effectiveness of reservoir fracturing and production enhancement.
Author Contributions
Funding
Data Availability Statement
Conflicts of Interest
References
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Test Name | Test Content | Test Result | |
---|---|---|---|
Analysis of maceral composition in coal sample | Demineralized bases (%) | Vitrinite | 78.75 |
Inertinite | 21.25 | ||
Exinite | - | ||
Mineral containing bases (%) | Total organic content | 29.41 | |
Clay | 5.80 | ||
Carbonate | 1.79 | ||
Average random reflectance (%) | 1.78 | ||
Standard deviation (%) | 0.068 | ||
Quantitative analysis of whole-rock X-ray diffraction | Calcite (%) | 30.5 | |
Dolomite (%) | 9.3 | ||
TCCM (%) | 60.2 | ||
Industrial and elemental analysis of coal samples | Cinder feature (1–8) | 4 | |
Proximate analysis (%) | Mad | 1.18 | |
Aad | 12.58 | ||
Vad | 14.80 | ||
FCad | 74.48 | ||
Ultimate analysis (%) | St,d | 0.30 | |
Cdaf | 90.13 | ||
Hdaf | 4.09 | ||
Odaf | 3.96 | ||
Ndaf | 1.48 |
Test No. | Coal Fine Parameters | Temporary Plugging | Unplugging | |
---|---|---|---|---|
Concentration (g/L) | Particle Size (Mesh) | |||
1 | No coal fines | No coal fines | Confining pressure: 3 MPa Plugging pressure: 2 MPa Plugging time: 60 min | Confining pressure: 3 MPa Unplugging velocity: 1 mL/min Unplugging time: 30 min |
2 | 3 | >180 | ||
3 | 5 | >180 | ||
4 | 7 | >180 | ||
5 | 10 | >180 | ||
6 | 5 | 120–150 | ||
7 | 5 | 80–100 |
Test No. | Coal Fine Parameters | Permeability (mD) | |
---|---|---|---|
Concentration (g/L) | Particle Size (Mesh) | ||
1 | No coal fines | No coal fines | 62.64 |
2 | 3 | >180 | 3.21 |
3 | 5 | >180 | 2.38 |
4 | 7 | >180 | 1.34 |
5 | 10 | >180 | 0.89 |
6 | 5 | 120–150 | 4.87 |
7 | 5 | 80–100 | 6.44 |
Test No. | Coal Fine Parameters | Permeability (mD) | |
---|---|---|---|
Concentration (g/L) | Particle Size (Mesh) | ||
1 | No coal fines | No coal fines | 62.64 |
2 | 3 | >180 | 32.12 |
3 | 5 | >180 | 19.27 |
4 | 7 | >180 | 46.84 |
5 | 10 | >180 | 55.07 |
6 | 5 | 120–150 | 9.96 |
7 | 5 | 80–100 | 13.19 |
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Wang, F.; Huang, F.; Guan, Y.; Xu, Z. Mitigation of Fracturing Fluid Leak-Off and Subsequent Formation Damage Caused by Coal Fine Invasion in Fractures: An Experimental Study. Processes 2024, 12, 1711. https://doi.org/10.3390/pr12081711
Wang F, Huang F, Guan Y, Xu Z. Mitigation of Fracturing Fluid Leak-Off and Subsequent Formation Damage Caused by Coal Fine Invasion in Fractures: An Experimental Study. Processes. 2024; 12(8):1711. https://doi.org/10.3390/pr12081711
Chicago/Turabian StyleWang, Fengbin, Fansheng Huang, Yiting Guan, and Zihan Xu. 2024. "Mitigation of Fracturing Fluid Leak-Off and Subsequent Formation Damage Caused by Coal Fine Invasion in Fractures: An Experimental Study" Processes 12, no. 8: 1711. https://doi.org/10.3390/pr12081711