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煤岩抗压强度和弹性模量对不同煤阶区煤层气开发的影响

Variation law of compressive strength and Young’s modulus of coal rock with coal rank and their significance for CBM development

  • 摘要: 为了分析煤阶控制下的煤岩抗压强度和弹性模量变化规律,提出不同煤阶区煤层气开发建议,通过岩石单轴压缩试验和煤质测试,获得了我国18个矿区20块煤岩样品的力学参数(抗压强度、弹性模量)和最大镜质体反射率Ro,max数据,综合前人部分测试数据,系统探讨了从低煤阶到高煤阶(Ro,max=0.33%
    ~3.44%)煤岩抗压强度和弹性模量的变化规律及其对煤层气压裂和排采的影响。结果表明:煤阶显著影响煤岩割理和孔隙发育,褐煤、次烟煤
    (Ro,max≤0.50%)割理不甚发育,但是孔隙度最大且以大孔为主。演化到中煤阶的过程中,煤岩孔隙度降低,中煤阶(Ro,max≥0.65%)之后,煤岩孔隙度略有增加并趋于稳定,但是割理逐渐发育直至Ro,max=1.50%左右达到最大割理密度。当煤岩进一步演化至高煤阶
    (Ro,max≤1.9%),煤大分子官能团重新聚合导致割理融退。受到煤阶控制,煤岩抗压强度和弹性模量随煤阶升高呈“M”型变化规律。在
    Ro,max≤0.5%(主要为褐煤、次烟煤)、Ro,max≥3.0%(主要为无烟煤)和1.0%≤Ro,max≤2.0%(主要为焦煤、瘦煤)时,煤岩抗压强度和弹性模量较小,容易起裂,但是裂缝短宽不易延伸,导流能力较差。可以提高压裂规模,增加裂缝延伸长度,排采应该注意控制速度,防止产能衰减过快;在0.5%≤Ro,max≤1.0%(主要为气煤、肥煤)和2.0%≤Ro,max≤3.0%(主要为贫煤)范围内时,煤岩抗压强度和弹性模量较高,需要较高起裂压力,但压裂缝易延伸,压裂时要控制压裂规模,防止压穿煤层顶底板隔水层,排采可以适当提高排采速度。

     

    Abstract: In order to analyze the variation law of compressive strength and Young′s modulus of coal under the control of coal rank, suggestions for coalbed methane development in different coal rank areas are put forward. Through rock uniaxial compression test and coal quality test, the mechanical parameters (compressive strength, Young's modulus) and Ro,max data of 20 coal rock samples from 18 mining areas in China were obtained, and some pre-test data were synthesized, based on which, the variation law of compressive strength and Young's modulus of coal rock from low coal rank to high coal rank (Ro,max=0.33%~3.44%) and its influence on fracturing and drainage were discussed systematically. The results show that coal rank significantly affects the development of coal rock cleats and pores. Lignite and sub-bituminous coal(Ro,max≤0.50%) have less developed cleats, but have the largest porosity and mainly macropores.. During the evolution to the middle coal rank, the porosity of the coal rock decreases. After the middle coal rank (0.65%≤Ro,max), the porosity of the coal rock slightly increases and stabilizes, but the cleat gradually develops to the maximum around Ro,max=1.50%. When coal rock further evolves to a high coal rank (1.90% ≤Ro,max), the coal macromolecular functional groups re-polymerize, resulting in the cleat retreat.. Under the control of coal rank, the compressive strength and Young's modulus of coal rock show an "M" -shaped variation law with the increase of coal rank. When Ro,max≤0.5% (mainly lignite, sub-bituminous coal),Ro,max≥3.0% (mainly anthracite) and 1.0%≤Ro,max≤2.0%(mainly coking coal, lean coal), the compressive strength and 〖JP2〗Young's modulus of coal rock are low and easy to crack, but the short and wide cracks are not easy to extend, and the conductivity is poor. It's necessary to appropriately improve the fracturing scale to increase the length of crack, and to control drainage speed to prevent the fast decay of production capacity. When 0.5%≤Ro,max≤1.0% (mainly gas coal, fat coal) and 2.0%≤Ro,max≤3.0% (mainly lean coal), the compressive strength and Young's modulus of coal rock are relatively high, which requires a high cracking pressure, but the crack is easy to extend. During the fracturing, the scale should be controlled to prevent it from penetrating through the water barrier on the roof and floor of the coal seam. Drainage speed should be appropriately improved as well.

     

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