Fracture Characteristics and Their Influence on Gas Seepage in Tight Gas Reservoirs in the Kelasu Thrust Belt (Kuqa Depression, NW China)
Abstract
:1. Introduction
2. Geological Setting
2.1. Tectonic Setting and Structural Evolution
2.2. Stratigraphy
2.3. Tight Gas Reservoirs
3. Samples and Methods
3.1. Samples
3.2. Methods
3.2.1. Counting Linear Density
3.2.2. Pressure Experiments on Fractured Core Samples
- Cut the core samples (Well KS205, 7204.9 m) to 5–6 cm long cylinders of 2.54 cm diameter.
- Put the core plug into the HPHT carbon fiber core holder and set the confining pressure at 15 MPa, then use the X-ray microscope to scan the core samples at 0 MPa pore pressure.
- Use the gas charging system to inject He gas into the core samples until the pore pressure reaches 2 MPa and 5 MPa respectively, and maintain each pressure for 48 h.
- Use the X-ray microscope to scan the core samples respectively at the two pore pressures above.
- Use Avizo software to interpret the three datasets from the 3D micro-CT scanner.
- Compare the fracture parameters interpreted by the micro-CT scanner with confining pressure of 15 MPa and pore pressures of 0 MPa, 2 MPa and 5 MPa, respectively.
3.2.3. Core Charging Experiment
4. Results
4.1. Fractures Classification
- Giant fractures (with length > 10 m and aperture width > 1 cm), which are recognized on outcrops and usually cut through sandstone members (Figure 5a,b).
- Macro-fractures (with length of 0.05–3 m and aperture width of 0.1–2 mm), which are recognized on cores and FMI logs (Figure 5c–e).
- Micro-fractures (with length of 0.05–2.5 mm and aperture width of 5–100 μm), which are recognized in thin sections and usually cut through grains (Figure 5f,g).
- Nano-fractures (with length circa. 50 μm and aperture width circa. 50 nm), which extend along the edges of grains (Figure 5h).
4.2. Characteristics of Macro- and Micro-Fractures
4.3. Effects of Abnormal High Fluid Pressure on Fracture Characteristics
4.4. Seepage Performance in Tight Gas Reservoirs
5. Discussion
5.1. Controlling Factors of Fracture Density
5.2. Contribution of Fractures to Permeability
5.3. Impact of Fractures on Gas Seepage
6. Conclusions
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
References
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Basin | Denver [45] | San Juan [45] | Appalachia [45] | Erdos | Sichuan | Kuqa |
---|---|---|---|---|---|---|
Gas field | Wattenberg | Blanco Mesaverde | Appalachian | Sulige | Guangan | Kelasu |
Formation | Muddy | Mesaverde | Clinton-Medina | Shihezi | Xujiahe | Bashijiqike |
Depth (m) | 2070–2830 | 1677–1900 | 1220–1829 | 2850–3600 | 2300–2650 | 6000–8000 |
Porosity (%) | 8–12 | 8–10 | 5–10 | 6–12 | 2–12 | 2.2–10 |
Permeability (10−3 μm2) | 0.05–0.005 | 0.5–2 | <0.1 | 0.88 | 0.38 | 0.57 |
Pressure coefficient | Abnormal low pressure | Abnormal low pressure | Abnormal low pressure | 0.83–0.89 | 1.2–1.5 | 1.65–2.27 |
Gas saturation (%) | 56 | 66 | 49 | 45–60 | 35–65 | 60–85 |
Gas yield (104 m3/day) | 2.0–5.2 | 0.57–0.96 | — | 2–5 | 0.5–4 | 22–117 |
Well | Depth (m) | Fracture Characteristics | Distance to Fault (km) | |||||
---|---|---|---|---|---|---|---|---|
Origin | Scale | Inclination Angle (°) | Apparent Length | Aperture | Linear Density 2 | |||
DB103 | 5677–5953 | Structural fracture | Macro- | 67–80 | 0.16–0.86 m | 0.2–1.12 mm | 1.79 piece/m | 0.1 |
Micro- | 22–81 | 0.25–2.50 mm | 12–50 μm | 4.37 piece/mm | ||||
KS2-1-6 | 6591–6822 | Macro- | 55–85 | 0.26–3.00 m | 0.10–0.35 mm | 0.39 piece/m | 0.5 | |
Micro- | 12–80 | 0.42–1.54 mm | 9–38 μm | 2.51 piece/mm | ||||
KS801 | 7048–7291 | Macro- | 65–83 | 0.30–0.57 m | 0.17–0.48 mm | 0.34 piece/m | 1.0 | |
Micro- | 25–86 | 0.05–1.09 mm | 6–15 μm | 1.62 piece/mm | ||||
DB202 | 5711–5956 | Macro- | 64–85 | 0.54–3.54 m | 0.12–1.51 mm | 0.6 piece/m | 1.8 | |
Micro- | 62–89 | 0.31–2.12 mm | 10–29 μm | 2.98 piece/mm | ||||
DB6 | 6857–6925 | Macro- | 44–72 | 0.05–0.22 m | 0.29–1.20 mm | 0.19 piece/m | 2.0 | |
Micro- | 20–82 | 0.02–1.43 mm | 7–19 μm | 0.85 piece/mm | ||||
KS9 | 7438–7580 | Macro- | 64–81 | 0.03–0.20 m | 0.23–0.55 mm | 0.24 piece/m | 2.5 | |
Micro- | 12–85 | 0.06–0.73 mm | 3–10 μm | 0.82 piece/mm |
No. | Well | Depth (m) | Lithology | Porosity (%) 2 | Matrix Permeability (10−3 μm2) 3 | Overall Permeability (10−3 μm2) 4 | Permeability Contribution from Fractures | Initial Flowing Pressure Gradient (MPa/cm) |
---|---|---|---|---|---|---|---|---|
1 2 | YH303 | 5200.4 | Sandstone | 21.4 | / | 521.41 | 0.00% | 0 |
2 | DB103 | 5842.6 | Tight sandstone | 6.5 | 0.08 | 178.66 | 99.99% | 0 |
3 | KS2-1-6 | 6715.9 | Tight sandstone | 5.8 | 0.05 | 7.23 | 99.98% | 0.06 |
4 | KS801 | 7151.3 | Tight sandstone | 7.6 | 0.02 | 2.61 | 99.98% | 0.13 |
5 | DB202 | 5861.5 | Tight sandstone | 1.8 | 0.06 | 15.98 | 99.98% | 0.05 |
6 | DB6 | 6874.4 | Tight sandstone | 3.1 | 0.06 | 0.17 | 64.71% | 0.32 |
7 | KS9 | 7526.8 | Tight sandstone | 1.2 | 0.01 | 0.04 | 75.00% | 0.41 |
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Dong, Y.; Lu, X.; Fan, J.; Zhuo, Q. Fracture Characteristics and Their Influence on Gas Seepage in Tight Gas Reservoirs in the Kelasu Thrust Belt (Kuqa Depression, NW China). Energies 2018, 11, 2808. https://doi.org/10.3390/en11102808
Dong Y, Lu X, Fan J, Zhuo Q. Fracture Characteristics and Their Influence on Gas Seepage in Tight Gas Reservoirs in the Kelasu Thrust Belt (Kuqa Depression, NW China). Energies. 2018; 11(10):2808. https://doi.org/10.3390/en11102808
Chicago/Turabian StyleDong, Yue, Xuesong Lu, Junjia Fan, and Qingong Zhuo. 2018. "Fracture Characteristics and Their Influence on Gas Seepage in Tight Gas Reservoirs in the Kelasu Thrust Belt (Kuqa Depression, NW China)" Energies 11, no. 10: 2808. https://doi.org/10.3390/en11102808