Study on the strain response characteristics of coal and rock borehole walls under high-pressure gas fracturing

There has been a lack of precise methods for monitoring and evaluating the entire process of gas fracturing. However, strain monitoring can effectively record the real-time initiation and propagation of cracks. By studying the strain response of borehole walls during high-pressure gas impact, the relationship between crack formation and strain response during the fracturing process can be clarified, enabling the identification of the optimal fracturing angle. A true triaxial experimental system for high-pressure gas fracturing of coal and rock was used, and experiments were conducted at five different impact angles (0, 30, 45, 60, 90°) to investigate crack morphology, pressure curves, and strain response characteristics of the borehole walls. The experimental results revealed that: ① As the impact angle increased, the crack morphology of coal and rock exhibited a pattern that was initially complex but later became simpler. ② The gas pressure during the fracturing process passed through four stages: an increase, a sharp drop, accumulation, and steady release. ③ The strain data for the borehole walls were predominantly tensile, and the strain curve displayed two distinct peaks: the first peak occurred 0.1 seconds after the pressure curve reached its peak, coinciding with the formation of the main crack; the second peak was generally associated with the propagation and expansion of the main crack. ④ When the impact angle was 45°, a more complex crack network tended to form within the specimen, resulting in the most effective fracturing.