基于孔壁应变发展规律的压裂孔三阶段起裂特征试验研究

利用自行研制的煤岩体水力压裂试验系统，开展了配比型煤与原煤水力压裂试验，测试并分析了水力压裂过程中压裂孔孔壁应变-水压曲线，并基于孔壁应变的发展规律，分析了压裂孔的三阶段起裂特征。结果表明，在压裂孔起裂过程中，钻孔孔壁呈现拉伸与压缩应变两种类型，并呈现拉伸破裂区与压缩变形区，其中压缩型应变具有较好的可恢复性，其应变恢复比远大于拉伸型应变；钻孔起裂过程分为3个阶段，即水气作用诱导微损伤形成阶段，孔壁内形成气流通道并产生初始损伤；局部损伤带形成阶段，孔壁形成拉伸破裂区和压缩变形区；试件失稳破坏阶段，裂缝不断延伸直至试件破裂，拉伸破裂区依然保持拉伸变形并较好地保持残余变形，而压缩变形区则由于作用力转向而得到一定程度恢复。研究成果对于揭示钻孔起裂行为及能量的演化规律具有重要理论意义。

Study of tri-stage fracturing characteristic of borehole based on strain of hole-wall in hydraulic fracturing process

Experiments are conducted on raw coal and reshaped coal samples at varied conditions using a self-developed hydraulic fracturing experimental equipment. The characteristic of strain of borehole wall is analyzed. The results show that the strain of borehole is obvious during the process of increasing water pressure, and the strain is difficult to be recovered while the water pressure is unloaded. There are two types of strains in the borehole, i.e. tensile strain and the compressive strain. There are two types of strain region during the hydraulic fracturing process. The compressive strain occurred in compression region is recovered sufficiently when the water pressure is unloaded. The hydraulic fracturing process is divided into three stages by strain curves, i.e. the water and gas induced micro-damage stage, the local damaged zones stage, and the unstable failure stage. At micro-damage stage, the initial damage stage is caused by the formation of gas stream channels within the borehole wall. At the next stages, the tensile and compressive regions are generated on the borehole wall. The tensile failure zone increases continually until the collapse of the borehole, while the compressive deformation zone has been recovered to some extent due to the rotational direction of the applied force. Therefore, the residual tensile strain is obvious when the borehole has fractured, but the residual compressive strain is unapparent. The study results are of interest in the field of crack initiation and energy evolution mechanism of borehole.