A New Tree-Type Fracturing Method for Stimulating Coal Seam Gas Reservoirs
Abstract
:1. Introduction
2. Synopsis of the New Tree-Type Fracturing Technology
2.1. Tree-Type Fracturing Method
2.2. Tree-Type Sub-Borehole Fracture Theory
2.3. Gas Seepage after Tree-Type Fracturing
3. Tree-Type Fracturing Laboratory Experiments
3.1. Experimental Apparatus
3.2. Experimental Procedures
- (1)
- The sample carrier was sealed with a load-bearing cover by using adhesive (Figure 3c) and then placed into the triaxial loading cavity and fixed by four mechanical loading plates (Figure 3d). The gas inlet and outlet pipes and the water injection pipe were then connected to the cover. Screens were inserted to make sure the sample was not in direct contact with the cover to ensure that the sample had full contact with seepage gas. The loading cavity was then closed and sealed.
- (2)
- Hydraulic oil was pumped into the loading cavity until the confining pressure reached the minimum horizontal principal stress σh. Then the axial and lateral stresses were loaded to vertical principal stress σv and the maximum horizontal principal stress σH by the servo pumps at a rate of 2 MPa/min. Next, but before fracturing was initiated, gas was injected into the sample to test sample permeability and to simulate pore pressure. Methane was used as the pore gas during testing. Gas was injected from the plane marked as “a” in Figure 3a and ejected from plane “c”. The injection pressure was 2.5 MPa [28]. When the flow of ejected gas was stable, the gas flow was used to calculate permeability [29,30]. The permeability experiments on Φ50 × 100 mm sandstone specimens taken from different directions showed that the sandstone has isotropic properties, so the permeability at a single direction can represent the permeability of sandstone [31].
- (3)
- After connecting the syringe pump to the simulated main borehole in the sample (by a steel tube), the real-time data acquisition system was started. The two pump plungers were filled with distilled water and then the pump was started to inject high-pressure water at the specified flow rate of 20 mL/min. The injection system pressurized until a rapid pressure drop, which indicated that the sample had fractured. The pump was shut off after the sample was fractured completely.
- (4)
- After fracturing, the water was drained and the gas injection system was started to conduct the gas seepage tests. Gas was injected from planes a, b and c, shown in Figure 3a, with the simulated main borehole serving as a gas outlet to measure stimulated gas production after fracturing.
3.3. Results
4. Tree-Type Fracturing Case Study
4.1. Test Site and Procedures
4.2. Results and Discussion
5. Conclusions
Acknowledgments
Author Contributions
Conflicts of Interest
References
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Sample Number | Triaxial Stresses σv/σH/σh (MPa) | Horizontal Stress Difference (MPa) | The Number of Sub-Boreholes | Initiation Pressure (MPa) | The Induced Stress (MPa) | Whether Fractures Are Induced? |
---|---|---|---|---|---|---|
#1 | 15/12/8 | 4 | 0 | 17.22 | ---- | ---- |
#2 | 15/12/8 | 4 | 3 | 15.24 | 5.69 | Yes |
#3 | 15/12/7 | 5 | 3 | 14.02 | 5.23 | Yes |
#4 | 15/12/6 | 6 | 3 | 10.18 | 3.80 | No |
#5 | 15/12/6 | 6 | 4 | 10.06 | 6.88 | Yes |
#6 | 15/12/6 | 6 | 6 | 9.25 | 7.64 | Yes |
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Li, Q.; Lu, Y.; Ge, Z.; Zhou, Z.; Zheng, J.; Xiao, S. A New Tree-Type Fracturing Method for Stimulating Coal Seam Gas Reservoirs. Energies 2017, 10, 1388. https://doi.org/10.3390/en10091388
Li Q, Lu Y, Ge Z, Zhou Z, Zheng J, Xiao S. A New Tree-Type Fracturing Method for Stimulating Coal Seam Gas Reservoirs. Energies. 2017; 10(9):1388. https://doi.org/10.3390/en10091388
Chicago/Turabian StyleLi, Qian, Yiyu Lu, Zhaolong Ge, Zhe Zhou, Jingwei Zheng, and Songqiang Xiao. 2017. "A New Tree-Type Fracturing Method for Stimulating Coal Seam Gas Reservoirs" Energies 10, no. 9: 1388. https://doi.org/10.3390/en10091388