Experimental investigation on the characteristics of fractures initiation and propagation for gas fracturing by using air as fracturing fluid under true triaxial stresses

•Experiments of gas fracturing were performed under true triaxial stresses.•Breakdown pressure of gas fracturing is higher than hydraulic fracturing.•Cracks furcation tend to occur in the propagation process of gas fractures.•Pressure fall-off curve of gas fracturing was flat, crack closure rate was slow.•Effect mechanism of fracturing fluid viscosity on gas fracturing was explained. Gas fracturing is a waterless fracturing technology, which can eliminate many disadvantages of hydraulic fracturing. Using air as fracturing fluid, large size physical model experiments of gas fracturing were performed under true triaxial stresses, the characteristics of the fracture initiation and propagation for gas fracturing under true triaxial stresses was studied, the similarities and differences between the gas fracturing and hydraulic fracturing under the same loading conditions were comparatively analyzed. The experiment results indicated that the P-T curves, acoustic emission characteristics, and fracture propagation morphology of gas fracturing are significantly different from hydraulic fracturing due to the differences in compressibility and viscosity of fracturing fluids. During fracturing by equivalent pumping rate under the same confining stress condition, the breakdown pressure of gas fracturing was larger than that of hydraulic fracturing. Both the pressure drop amplitude and rate after the fracture initiation of gas fracturing were less than that of hydraulic fracturing. The pressure fall-off curve of gas fracturing was relatively flat, and the fracture closure rate was slow during the process of measuring the pressure drop after stopping the pump. While the pressure fall-off curve of hydraulic fracturing was relatively steep, and the fracture closure rate was fast. The acoustic emission energy in the initial rupture during the gas fracturing process was significantly higher than that of hydraulic fracturing; the moisture would cause reduction of the acoustic emission energy. Because the rupture during hydraulic fracturing lasted for a longer time and released more energy, the cumulative energy of the acoustic emission during the gas fracturing was lower than that of hydraulic fracturing. When fracturing by equivalent pumping rate for same time under the same confining stress condition, the propagation range of gas fractures was smaller than hydraulic fractures. Cracks furcation tends to occur in the propagation process of gas fractures. The surface roughness