Natural fractures widely exist in shale reservoirs with different scales, ranging from nano scale to micro scale, even to macro and engineering scale. These multi-scale natural fractures have a great influence on the fracture network propagation during a fracturing treatment. Meanwhile, the fracturing fluid flowing into these natural fractures will result in the co-existence of fracturing fluid and natural gas. As a result, the capillary force occurs at the gas-liquid interface in natural fractures. If the sizes of these natural fractures are different, the capillary force in the natural fractures is different as well. In particular, when the natural fracture is small, the capillary force will be very large and even affect the natural fracture initiation and propagation. As a consequence, the fracturing fluid flowing in natural fractures and the fracture network development will be significantly affected by these multiscale natural fractures. In this paper, a model about fracture network propagation was built which considered natural fractures sizes and fracturing fluid wettability. Besides, different methods were adopted to calculate the stress intensity of hydraulic fracture and natural fracture, respectively. In addition, the competitive growth theory of multi fractures is considered to describe the fracture network propagation. The studies found that with long length, small width natural fractures and strong wettability fracturing fluid, natural fractures were more likely to propagate, which resulted in a large fracture network. Besides, when the sizes of natural fractures were different, a more complex fracture network was tended to come out. An experimental study about fracture network propagation in shale was introduced from Suarez-Rivera’s work, in which a “fish bone” shape fracture propagation was observed. By comparing the numerical simulation with Suarez-Rivera’s work, it could be concluded that the simulation was in accordance with shale fracturing experiments results.
国家自然科学基金(51490650, 51490651)
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Figure 1
(Color online) The nanoscale and microscale natural fractures in shale.
Figure 2
(Color online) Schematic of the fluid and pressure distribution in a natural fracture after a hydraulic fracture crosses.
Figure 3
(Color online) Effect of natural fracture width on capillary force. (a) Wetting angle=30°; (b) surface tension=50?mN/m.
Figure 4
(Color online) Fracture network propagation of same size natural fractures after hydraulic fracturing.
Figure 5
(Color online) The influence of natural fractures initial half-length on fracture network propagation. (a) The initial half-length of natural fractures are 1?m; (b) the initial half-length of natural fractures are 0.5?m.
Figure 6
(Color online) The influence of natural fractures width on fracture network propagation. (a) The width of natural fractures are 30?nm; (b) the width of natural fractures are 40?nm.
Figure 7
(Color online) The influence of fracturing fluid wetting angle on fracture network propagation. (a) The wetting angle between fracturing fluid and reservoir is 30°; (b) the wetting angle between fracturing fluid and reservoir is 20°.
Figure 8
(Color online) The influence of Gas-liquid interfacial tension on fracture network propagation. (a) The surface tension of fracturing fluid and shale gas is 50?mN/m; (b) the surface tension of fracturing fluid and shale gas is 40?mN/m.
Figure 9
(Color online) Schematic of multi-scale fracture network propagation.
Figure 10
(Color online) Experimental study on fracture network propagation.
序号 | 裂缝参数 | NF1 | NF2 | NF3 | NF4 | NF5 | NF6 | NF7 | NF8 |
a | 0.5 | 1 | 1 | 1.5 | 1 | 1 | 1.5 | 1 | |
0.5 | 1 | 1 | 1.5 | 1 | 1 | 1.5 | 1 | ||
30 | 30 | 30 | 30 | 30 | 30 | 30 | 30 | ||
b | 0.7 | 0.8 | 1 | 1.5 | 0.6 | 1 | 1.2 | 1 | |
0.5 | 0.7 | 1 | 1 | 1 | 1 | 1.4 | 1 | ||
30 | 30 | 30 | 30 | 30 | 30 | 30 | 30 | ||
c | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |
1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | ||
40 | 30 | 30 | 20 | 30 | 40 | 20 | 30 | ||
d | 0.7 | 1 | 1 | 2 | 1 | 0.8 | 1 | 1.2 | |
0.7 | 1 | 1 | 2 | 1 | 0.8 | 1 | 1.2 | ||
40 | 30 | 27 | 20 | 40 | 37 | 27 | 30 | ||
e | 0.6 | 1.2 | 1 | 1 | 1 | 0.9 | 0.9 | 1.2 | |
0.8 | 0.8 | 1 | 1.5 | 1 | 0.7 | 1.2 | 1.2 | ||
40 | 30 | 30 | 20 | 40 | 30 | 25 | 30 | ||
f | 0.5 | 1 | 1 | 2.2 | 1 | 0.8 | 1 | 1.3 | |
0.7 | 1 | 1.5 | 2.2 | 1 | 0.8 | 1.5 | 1.3 | ||
40 | 30 | 27 | 17 | 40 | 37 | 27 | 30 |
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