煤储层水力压裂是提高煤层气井产量的关键技术，水力压裂前后煤储层渗透率的变化反映了煤储层压裂改造效果。采用山西晋城矿区寺河煤矿二叠系山西组3号煤层高煤阶试样，通过柱型煤样水力压裂前后渗透率试验对比分析，测试了4个高煤阶煤样水力压裂前后渗透率分布特征，揭示了水力压裂前后煤样渗透率随应力的变化规律和控制机理。结果表明，在围压和轴压恒定条件下，煤样孔隙压力随注入压力增大逐渐增高；当注入压力大于破裂压力时，煤样发生破裂，煤样的破裂压力随围压的增加呈线性增大的规律。在注入压力相同的情况下，随着围压和轴压的增大，有效应力增高，水力压裂前后煤样渗透率随有效应力的增大呈指数函数关系减小，且压裂后的渗透率要明显大于压裂前的渗透率。采用煤储层渗透率改善率来评价围压下煤样水力压裂增渗效果，4个试验煤样渗透率改善率随有效应力的增高呈指数函数关系增大，但随围压的增大，渗透率改善率提升幅度逐渐降低。煤储层的渗透性主要取决于煤中裂隙发育程度和裂隙开度的大小，裂缝储层的渗透率的大小与裂缝张开度的3次方成正比例关系。水力压裂后裂缝平均长度、裂缝孔隙度和裂缝开度增幅分别为70.81%～253.25%、171.88%～383.02%和20.31%～32.43%，且水力压裂前后煤样裂缝开度均随有效应力的增高而呈负指数函数规律降低。试验结果与实测水力压裂前后煤储层渗透率对比，两者具有较好的一致性，为煤储层水力压裂改造效果评价探索了有效途径。

Hydraulic fracturing technology has become a key technical means to improve coalbed methane (CBM) yield. The evolution of CBM reservoir permeability before and after hydraulic fracturing reflects the induced fracturing level. By means of fracturing and stresspermeability experiments on four high rank coal samples of No.3 coal seam of the Permian Shanxi Formation in Sihe Coal Mine, Jincheng Mining Area, Shanxi Province, the permeability changes before and after fracturing are tested, and the evolution of pillar coal sample permeability with stress before and after fracturing is revealed. The results show that under the condition of constant confining pressure and axial pressure, the pore pressure of coal samples increases gradually with the increase of injection pressure. When the injection pressure is greater than the fracture pressure, the coal sample breaks, and the fracture pressure of the coal sample increases linearly with the increase of the confining pressure. Under the same injection pressure, the effective stress increases with the increase of confining pressure and axial pressure. The permeability of coal samples before and after hydraulic fracturing decreases exponentially with the increase of effective stress, and the permeability after fracturing is significantly greater than that before fracturing. The permeability improvement coefficient (PIC) of CBM reservoir is used to evaluate the permeability improvement effect of coal samples. The PIC of four test coal samples increase exponentially with the increase of effective stress, but decrease with the increase of confining pressure. The permeability of CBM reservoir mainly depends on the degree of fracture development and the size of fracture width in coal. The permeability of fractured reservoir is in direct proportion to the 3rd power of fracture width. After hydraulic fracturing, the average fracture length, fracture porosity and fracture width increase by 70.81%−253.25%, 171.88%−383.02% and 20.31%−32.43%, respectively. The fracture width decreases with the increase of effective stress in a negative exponential function. The test results are compared with the measured coal reservoir permeability before and after hydraulic fracturing. The two have good consistency, which explores an effective way for the evaluation of the effect of hydraulic fracturing of coal reservoirs.

0 引言 1 试验条件与方法 1.1 试验样品 1.2 试验条件 1.3 试验方法及步骤 2 试验结果及分析 2.1 煤样破裂压力 2.2 水力压裂前后煤样应力-渗透率变化特征 3 讨论 3.1 水力压裂后煤样增渗效果评价 3.2 水力压裂前后煤样渗透率的受控机制 4 结论