Mechanical behavior and fracture spatial propagation of coal injected with liquid nitrogen under triaxial stress applied for coalbed methane recovery

Fracturing technology through liquid nitrogen (LN2) injection alters the physical properties of coal and rock masses because of the thermal and pressure effects. In order to investigate heat transfer and fracture propagation behaviors under in situ geological condition with LN2 injections, experimental work was conducted to study the LN2 induced rock/coal failures under true triaxial stress conditions. During the experiments, the temperature, ultrasonic waves, and acoustic emission location detection were monitored to determine the intensity and complexity of specimen failure under singular or cyclic LN2 injections. The results show that a single LN2 injection mainly transfers heat through the solid skeleton and only damages areas adjacent to the injection tube. However, cyclic injections formed a propagated fracture network and the heat can sequentially transfer further along the induced fractures. Moreover, plastic deformation occurred in the entire volume of the sample and the main fractures coalesced until the sample failed. Based on the spatial locations of the acoustic emission sources, the dynamic fracturing within the sample progressed after LN2 injection was clarified. The experimental results can provide evidences for the proposed crack propagation model for the coal masses. The research revealed that high-pressure nitrogen gas transferring liquid water to the tips of new fractures is essential for cyclic LN2 injection to form effective frost-heaving forces and fracturing. Therefore, the fracturing efficiency of the cyclic LN2 injection is far higher than that of singular LN2 injections.