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液N_2等气体辅助煤层气压裂的常规机理已较为清楚,但其低温特征对煤层物性的影响以及对压裂效果的改善机理尚未引起重视,缺乏理论认识.归纳总结了低温气体对煤岩的冷冲击作用机制,评价了地层水的结冰条件和低温气体的热物性,通过室内实验和数值模拟等手段验证和预测了液N_2对煤岩及近井煤层的冷冲击效果,并进行了冷冲击机理应用潜力分析.研究结果表明,煤层气压裂过程中注入N_2等低温气体对煤层进行冷冲击,可引起煤岩基质收缩和地层水结冰膨胀,使煤岩产生大量微裂缝和力学强度降低(10%~30%),有利于实现冰晶暂堵和改善煤层气压裂效果.以沁水盆地樊庄区块3#煤层为例,当液N_2注入总量为30~120m~3时,可在近井周围3~5m内形成低温区(<-20℃),煤层孔隙度将平均增大约1.5倍,渗透率增大4倍,还可造成煤岩的拉伸和挤压破坏.利用低温气体的冷冲击机理及其产生的冰晶暂堵可以作为改进煤层气和页岩气压裂工艺的新方向.
Although the conventional mechanism of gas-assisted gas-assisted CBM fissure such as liquid N_2 has been relatively clear, the influence of low-temperature characteristics on the physical properties of the coalbed and the improvement mechanism of fracturing effect have not been given much attention and lack of theoretical understanding. Cold impact mechanism, the icing conditions of formation water and the thermophysical properties of cryogenic gas were evaluated. The cold impact of liquid N 2 on coal and near-well coal seam was verified and predicted by laboratory experiments and numerical simulation. Impact mechanism of the application potential.The results show that the injection of N 2 and other low-temperature gas into the coal seam during the CBM fracturing process can cause the rock matrix to shrink and the formation water to freeze and expand, resulting in a large number of micro-cracks and mechanical strength (10% -30%), which is in favor of temporary plugging of ice crystals and improving the fracturing effect of coalbed methane.Based on the No.3 seam of Fanzhuang block in Qinshui Basin, when the total amount of liquid N_2 is 30 ~ 120m ~ 3 , Low temperature zone (<-20 ℃) can be formed within 3 ~ 5m near the well. The porosity of coal seam will increase about 1.5 times and permeability increase 4 times on average, but also lead to the tensile and crush of coal and rock. Cold shock mechanism using low temperature gas and its production The raw ice temporary plugging can be used as a new direction to improve the CBM and shale gas fracturing process.