Analysis on the Initial Cracking Parameters of Cross-Measure Hydraulic Fracture in Underground Coal Mines
Abstract
:1. Introduction
- (1)
- Hydraulic fracturing in underground coalmines is an auxiliary technology, serving subsequent coal mining. Therefore, monotonic increase of the pump pressure during the fracturing process is not allowed to avoid damaging the integrity of coal seam roof and floor. In this aspect, Lin studied the pulsating stress wave generation, propagation and the mechanism of coal and rock breakage from the theoretical perspective [17].
- (2)
- The coal bed is thin and bounded by coal seam roof and floor, and it is conventionally assumed that the physical and mechanical properties of coal would not affect hydraulic fracturing initiation. However, hydraulic fracturing initiation can be influenced by other factors, such as in-situ stress and rock strength. Using numerical simulation, Lian and Lin, et al. found the rule of crack initiation and propagation influenced by in-situ stress and rock strength [18,19].
- (3)
- The layout of a borehole is limited by the occurrence of coal seam such as strike, inclination, and dip angle as shown in Figure 1.
2. Model Development
2.1. Stress Analysis for Cross-Measure Fracturing Boreholes
2.2. Model for Calculation of Initial Cracking Parameters
3. Field Experimentation
3.1. Mine-Site Condition and Borehole Layout
Number | Borehole Size/mm | Azimuth Angle/° | Borehole Angle/° | Borehole Depth/m |
---|---|---|---|---|
1# | 91 | 0 | 60 | 30.2 |
2# | 91 | 0 | 57 | 33.2 |
3# | 91 | 0 | 55 | 35.1 |
4# | 91 | 0 | 53 | 37.0 |
5# | 91 | 0 | 50 | 39.8 |
6# | 91 | 0 | 47 | 42.4 |
7# | 91 | 0 | 44 | 45.0 |
8# | 91 | 0 | 42 | 46.6 |
3.2. In-Situ Stress Measurement in Test Site
Equipment Name | Type | Parameters |
---|---|---|
Electronic precision material testing machine | AG-I 250kN | 1/1000 degree loading precision, 1/1000 degree displacement control precision |
Acoustic emission detection system | 16CHsSAMOS™ System | 12 channels, 40 MHz, 18 bit A/D, 1 KHz–3 MHz frequency range, 100 dB maximum signal amplitude, 40 MSPS sample rate |
Category | The Maximum σ1 | The Intermediate σ2 | The Minimum σ3 |
---|---|---|---|
Principal stress /MPa | 23.7 | 9.9 | 4.5 |
θx (The angle between principal stress and x)/° | 39.0 | 94.6 | 51.1 |
θy (The angle between principal stress and y)/° | 129.1 | 87.8 | 38.9 |
θz (The angle between principal stress and z)/° | 86.8 | 3.4 | 88.4 |
3.3. Hydraulic Fracturing Equipment for Field Test
4. Results and Discussion
4.1. Analysis on the Initiation Parameters
4.1.1. State of In-Situ Stress in Coal Seams
4.1.2. The Effect of Burial Depth on Borehole Cracking Initiation
4.1.3. The Effect of the Coal Seam Dip Angle on Borehole Cracking Initiation
4.1.4. The Effect of the Stress Orientations on Boreholes Initiation
4.2. Field Validation and Discussion
Number | 1# | 2# | 3# | 4# | 5# | 6# | 7# | 8# |
---|---|---|---|---|---|---|---|---|
Initiation pressure/MPa | 16.8 | 17.3 | 17.9 | 18.4 | 19.7 | 20.8 | 21.9 | 22.6 |
Initiation location/° | 144 | 143.5 | 143.1 | 142.7 | 142 | 141.2 | 140.6 | 139.7 |
- (1)
- The fracturing pump is far away from the borehole for the safety purpose and a certain amount of pressure is dissipated in the pipe line from the pump to borehole destination. This amount of pressure loss can be calculated using the following Equation [30]:
- (2)
- The gravity of water increases the actual initiation pressure due to the height difference between fractured borehole bottom and the discharged pump. This amount of pressure loss can be calculated from the following Equation:
5. Conclusions
Acknowledgments
Author Contributions
Conflicts of Interest
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Lu, Y.; Cheng, L.; Ge, Z.; Xia, B.; Li, Q.; Chen, J. Analysis on the Initial Cracking Parameters of Cross-Measure Hydraulic Fracture in Underground Coal Mines. Energies 2015, 8, 6977-6994. https://doi.org/10.3390/en8076977
Lu Y, Cheng L, Ge Z, Xia B, Li Q, Chen J. Analysis on the Initial Cracking Parameters of Cross-Measure Hydraulic Fracture in Underground Coal Mines. Energies. 2015; 8(7):6977-6994. https://doi.org/10.3390/en8076977
Chicago/Turabian StyleLu, Yiyu, Liang Cheng, Zhaolong Ge, Binwei Xia, Qian Li, and Jiufu Chen. 2015. "Analysis on the Initial Cracking Parameters of Cross-Measure Hydraulic Fracture in Underground Coal Mines" Energies 8, no. 7: 6977-6994. https://doi.org/10.3390/en8076977