不同层理角度对煤岩水力裂缝扩展的影响分析

    Analysis of the influence of different bedding angles on hydraulic fractures propagation in coal rock

    • 摘要: 为探究不同层理角度对煤岩水力压裂裂缝扩展的影响,采用零厚度Cohesive 黏结单元的损伤准则,建立了不同层理角度(0°、30°、45°、60°、90°)煤岩水力压裂裂缝扩展的有限元模型,通过水力裂缝扩展形态、分支缝的条数、缝宽来表征不同层理角度的水力压裂结果;分析了不同层理角度煤岩分别在不同水平地应力差下的水力裂缝宽度和地层内孔隙压力大小,并在神府区块FG-T29井进行了验证。结果表明:层理角度为0°时,水力裂缝穿透了第1层层理后扩展;层理角度为30°时,裂缝穿过第1层层理后,沿第2层层理方向延展,随后在层理处出现转向;层理角度为45°时,裂缝沿着第1层理延展后发生转向;60°、90°层理的裂缝均没有扩展到层理面;与其他4种层理角度相比,45°层理煤岩在压裂过程中出现的分支缝条数最多(15条)、缝宽最大(6.19 mm);随着水平地应力差增大,5种层理角度煤岩的水力裂缝缝宽均减小,起裂压力均增大。

       

      Abstract: To explore the impact of different bedding angles on the fractures propagation of coal hydraulic fracturing in coal-rock formations, based on the zero thickness Cohesive unit damage criterion, a finite element model of coal and rock hydraulic fracturing fractures propagation at different bedding angles (0°, 30°, 45°, 60°, 90°) is established. The hydraulic fracturing results at different bedding angles are characterized by hydraulic fracture propagation morphology, the number of branch fractures and joint widths. The hydraulic fracture width and pore pressure of coal rock under different horizontal geo-stress difference at different bedding angles are analyzed and verified in well FG-T29 of Shenfu Block. The results show that when the bedding is 0°, the hydraulic fracture spreads after penetrating the first bedding; when the bedding is 30°, the fractures first pass through the first bedding, extend along the direction of the second bedding, and then turn at the bedding; when the bedding is 45°, the fractures turn after extending along the first layer; the cracks of 60° and 90° bedding did not extend to the bedding surface. Compared with the other 4 bedding angles, 45° bedding coal rock has the largest number of branch fractures (15) and the largest fracture width (6.19 mm) during fracturing. With the increase of horizontal geo-stress difference, the crack width of hydraulic fracture decreases and the initiation pressure increases for all the five bedding angles.

       

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