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 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.