水力压裂煤裂隙网络表征与造缝性能评估试验研究

Experimental study on characterization hydraulic fracturing coal fracture network and evolution of fracture forming performance

煤裂隙网络的准确表征可以有效评估深部煤层经水力压裂后的压裂效果,为了定量评估煤层经水力压裂后的复杂程度,利用自制的真三轴试验系统进行了煤的水力压裂试验,结合CT扫描,重建了具有拓扑结构的孔裂隙网络,用分形理论和拓扑学定量表征了断裂网络的复杂程度。探究了在真三轴应力条件下,中间主应力对裂缝网络复杂程度的影响。结果表明:压裂后煤样的二维分形维数变化率K为1.03%~7.10%,三维分形维数的变化率为3.50%~9.18%,经过水力压裂后煤样的二维和三维分形维数均显著增大。基于分形理论和拓扑学的方法能有效表征水力压裂的裂缝结构和造缝能力。压裂后煤样的二维拓扑参数为1.18~1.52,与压裂前后煤样的二维分形维数变化率呈正相关关系,二维分形维数变化率的增加速率随着拓扑参数的增大逐渐减小。重构的内部结构和裂缝分布表明,压裂后的三维分形维数比二维分形维数更具优势,压裂后煤样的三维拓扑参数为1.82~1.93,随着三维分形维数变化率的增加而增大,三维分形维数变化率的增加速率随着拓扑参数的增大而增大。中间主应力对煤层的造缝能力存在积极影响。水力压裂前后的分形维数和拓扑参数都随着中间主应力的增大而增大,即随着中间主应力的增大而增大,产生的裂缝网络更复杂,连通性更好,水力压裂的造缝能力更强。

The accurate characterization of coal fracture network can effectively evaluate the fracturing effect of the deep coal seam after hydraulic fracturing.In order to quantitatively evaluate the complexity of coal seam after hydraulic fracturing,the hydraulic fracturing test of coal was carried out by using the self-made true triaxial test system,combined with CT scan,the pore network and fracture network with topological structure are reconstructed,and the complexity of the fracture network is quantitatively characterized by fractal theory and topology.The effect of the intermediate principal stress on the complexity of the fracture networks under the condition of true triaxial stress is explored.The results showed that the change rate of the two-dimensional fractal dimension(K)are 1.03%~7.10%,and the change rate of three-dimensional fractal dimension are 3.50%~9.18%,the two-dimensional and three-dimensional fractal dimensions of coal samples after hydraulic fracturing increase significantly.The method based on fractal theory and topology can effectively characterize the fracture structure and fracture forming ability of hydraulic fracturing.The two-dimensional topological parameter of coal samples after fracturing are1.18~1.52,which is positively correlated with the change rate of two-dimensional fractal dimension.Increasing rate of change rate of two-dimensional fractal dimension decreases with the increase of topological parameters.The reconstructed internal structure and fracture distribution showed that the three-dimensional fractal dimension after fracturing is more advantaged than the two-dimensional fractal dimension.The three-dimensional topological parameters of coal samples after fracturing are 1.82-1.93,increased with the increase of the change rate of three-dimensional fractal dimension.The intermediate principal stress has a positive effect on the seam forming ability of coal seam.The fractal dimension and topological parameters increase with the increase of intermediate principal stress before and after hydraulic fracturing.In other words,Increased with the intermediate principal stress,the resulting fracture network is more complex,the connectivity is better,and the fracture forming ability of hydraulic fracturing is stronger.