Experimental and numerical study on critical oxygen concentration for shale gas detonation induced by oxygen-enriched combustion

•Oxygen-enriched combustion-induced detonation (OEC-DET) was first discovered.•OEC-DET is ascribed to H2O phase transition and the promoted pressure wave development.•Critical oxygen concentration (COC) of OEC-DET in shale gas is around 25 %–30 %.•The lower the nitrogen content and equivalence ratio, the lower the COC.•Fast prediction of COC with more than 90% accuracy was achieved. To improve the shortcomings of Shchelkin spiral in enhancing overpressure utilizing oxygen-enriched combustion-induced detonation (OEC-DET) has seldom been touched upon. Systematic experiments and simulations have been conducted to reveal the kinetic mechanism and critical conditions of OEC-DET. Longmaxi (LMX) and Cambrian (HWX) shale gases with 4 % and 20 % nitrogen content were selected as fuels. Explosion overpressure histories of 0.8, 1.0 and 1.2 equivalence ratio fuels at different oxygen concentrations were tested in a 90 L multiphase fuel detonation tube. Temperature sensitivity coefficients of combustion reactions were then supplemented by kinetic simulations. OEC-DET was demonstrated to be a manifestation of pressure wave development promoted by liquid–gas phase transition of H2O produced by OEC. One underlying mechanism is that the increase in oxygen concentration enhances the temperature sensitivity and H2O production rate, thus favoring the occurrence of OEC-DET. The critical oxygen concentration (COC) of OEC-DET in shale gas has been determined to be around 25 %–30 % herein. The lower the nitrogen content and equivalence ratio, the lower the COC. Both the explosion overpressure and pressure rise rate exhibit segmented linear and exponential growth with the increase of COC, and particularly the growth trend becomes accelerated after OEC-DET. Two COC prediction equations have thus been derived with an accuracy exceeding 90 % and these shed light on the thermal engineering such as in-situ methane deflagration fracturing.