Effect of in-situ stress on the cracking and permeability enhancement in coal seams by deep-hole cumulative blasting
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摘要: 針對地應力對煤層深孔聚能爆破致裂增透問題，在分析鉆孔圍巖應力場、爆生裂隙擴展及動態卸載效應的基礎上，對不同地應力條件下聚能爆破作用過程及裂隙發育特征進行了數值模擬，并通過在不同埋深下的聚能爆破現場試驗，探討了地應力對煤層深孔聚能爆破致裂增透的作用。結果表明：在高地應力煤層進行深孔聚能爆破時，地應力在煤層深孔聚能爆破裂隙擴展不同階段的作用存在較大區別，在未進行聚能爆破時，鉆孔圍巖應力狀態及形變特征由鉆孔形態以及地應力共同決定。在聚能爆破作用初始階段，由于聚能爆破對圍巖產生的沖擊作用明顯強于地應力，因此爆生裂隙在初期的擴展方向主要由聚能裝藥結構控制，沿聚能槽開口方向形成定向裂隙；隨著裂隙向四周擴展，爆破作用逐漸減弱，地應力作用逐漸顯現，鉆孔圍巖在地應力作用下產生切向壓應力，限制了爆破徑向裂隙擴展。同時，與主應力方向不同的煤體裂隙在較強的剪應力作用下逐漸沿最大主應力方向偏轉。當爆破作用產生的等效動態應力無法繼續使煤體進一步壓縮時，鉆孔圍巖內積聚的彈性應變能開始朝爆破中心方向釋放，形成新的裂隙。此外，不同方向上的裂隙擴展范圍受側壓系數控制，當垂直主應力一定時，隨著側壓系數增大，最小主應力方向的裂隙范圍進一步減小。Abstract: With the gradual development of coal mining to deeper levels, the in-situ stress of coal seams shows an increasing trend, resulting in a gradual decrease in permeability, and the stress state of the coal and rock mass and the properties of the surrounding rock also change. The mechanical properties and mechanical parameters of coal and rock mass greatly differ between depths, which influences the cracking and permeability enhancement effect of coal seam deep-hole cumulative blasting. Aiming at the problem of the increasing permeability of coal seams by deep-hole cumulative blasting under in-situ stress, on the basis of an analysis of the stress field of the surrounding rock and the stress of the blasting crack surface, the process of cumulative blasting and crack development characteristics under different confining pressures were numerically simulated. Through field tests of cumulative blasting under different buried depths, the influence of in-situ stress on the cracking and permeability enhancement effect of coal seam deep-hole cumulative blasting was discussed. The results show that the role of in-situ stress differs greatly between the stages of radial crack expansion of coal seam deep-hole cumulative blasting. Before blasting, the stress state and deformation characteristics of the borehole surrounding rock are determined by borehole shape and in-situ stress. In the initial stage of cumulative blasting, the impact of cumulative blasting on the surrounding rock is obviously stronger than in-situ stress. Therefore, the expansion direction of blasting cracks in the initial stage is mainly determined by the cumulative structure, and directional cracks are formed along the opening direction of the cumulative charge groove. With the crack extension, the blasting effect is gradually weakened, and the in-situ stress is dominant. The surrounding rock of the borehole produces tangential compressive stress under the in-situ stress, which limits the radial crack expansion of blasting. Meanwhile, the coal cracks that are not collinear with the principal stress gradually deflect toward the direction of the maximum principal stress under the action of strong shear stress. When the equivalent dynamic stress produced by blasting cannot continue to compress the coal, the elastic strain energy accumulated in the surrounding rock of the borehole begins to release toward the blasting center, causing the coal to crack and produce new cracks. In addition, the crack expansion range in different directions is controlled by the lateral pressure coefficient. When the vertical principal stress is constant, the crack range toward minimum principal stress further decreases with increasing lateral pressure coefficient.
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圖 2 鉆孔受力情況. (a) 圍巖受力情況; (b)單元受力情況
Figure 2. Stress diagram of the borehole: (a) stress condition of surrounding rock; (b) stress condition of unit
圖 3 地應力作用下鉆孔圍巖塑性區范圍
Figure 3. Range of the plastic zone of the borehole surrounding rock under in-situ stress
圖 7 煤體爆生裂隙發育特征. (a) σv=10 MPa, λ=1; (b) σv=10 MPa, λ=1.5; (c) σv=10 MPa, λ=2;(d) σv=20 MPa,λ=1; (e) σv=20 MPa, λ=1.5; (f) σv=20 MPa, λ=2
Figure 7. Development characteristics of coal cracks formed by blasting: (a) σv = 10 MPa, λ = 1; (b) σv = 10 MPa, λ = 1.5; (c) σv = 10 MPa, λ = 2; (d) σv=20 MPa, λ = 1; (e) σv = 20 MPa, λ = 1.5; (f) σv = 20 MPa, λ = 2
圖 8 聚能爆破裂隙發育過程. (a) t=268 μs; (b) t=600 μs; (c) t=1000 μs
Figure 8. Crack development process of cumulative blasting: (a) t = 268 μs; (b) t = 600 μs; (c) t = 1000 μs
圖 10 平煤股份十礦、十二礦地應力隨埋深關系
Figure 10. Relationship between in-situ stress and buried depth in the No.10 and No.12 coal mines of Pingdingshan
圖 11 煤層深孔聚能爆破試驗鉆孔布置示意圖. (a) 順層鉆孔；(b) 穿層鉆孔
Figure 11. Layout of the boreholes of deep-hole cumulative blasting in a coal seam: (a) borehole drilling along the seam; (b) borehole drilling across the seam
圖 12 煤層深孔聚能爆破后抽采孔內瓦斯體積分數及純流量變化. (a) 埋深 ≈ 837 m; (b) 埋深 ≈ 943 m; (c) 埋深 ≈ 1057 m
Figure 12. Variation in gas volume fraction and flow rate in the drainage hole before and after cumulative blasting: (a) buried depth ≈ 837 m; (b) buried depth ≈ 943 m; (c) buried depth ≈ 1057 m
圖 13 地應力作用下的聚能爆破煤體致裂過程. (a) 鉆孔初始受力階段; (b) 聚能爆破作用主控階段; (c) 地應力作用主控階段; (d) 圍巖動態卸載效應主控階段
Figure 13. Coal cracking process of cumulative blasting under in-situ stress: (a) initial stress stage of borehole; (b) main control stage of cumulative blasting; (c) main control stage of in-situ stress; (d) main control stage of surrounding rock dynamic unloading effect
中文字幕在线观看表 1 試驗區鉆孔遠場圍巖應力狀態
Table 1. Stress state of the surrounding rock of the borehole in the test area
Experimental
locationBuried
depth/mAzimuth/
(°)Dip angle/
(°)σh/MPa σv/MPa λ 24100
Working face≈ 837 22.2 13 26.9 21.7 1.24 31060
Working face≈ 943 34.5 35.5 32.3 23.4 1.38 33200
Working face≈ 1057 24.0 12 30.9 26.4 1.17 
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