Experimental study on crack propagation of coal-rock mass under supercritical CO2 fracturing

In order to grasp the law of the initiation and the crack propagation in fracturing coal-rock mass with supercritical CO2(SC-CO2),the experimental study was carried out by using TCHSFM-I large-scale true-triaxial simulation device for fracturing and seepage and by collecting pressure and surface cracks data ,combined with theoretical analysis. The results showed that the pressure-time curves of fracturing coal-rock with supercritical CO2 could be divided into four stages:cavity-filling stage,pore pressurization stage,phase-transiting pressurization stage and destabilization failure stage.The initiation pressure values of coal and sandy mudstone initiation were 12.32 MPa and 19.63 MPa respectively.The coal,with its Biot coefficient of 0.91,brittleness coefficient of 16.95% and elastic modulus of 3.48 GPa,produced horizontal cracks parallel to the bedding; while the sandy mudstone,with its Biot coefficient of 0.69,brittleness coefficient was 47.53% and elastic modulus of 10.52 GPa,produced connective and complex cracks perpendicular to the bedding. It could be seen that the initiation of fracturing with SC-CO2 fracturing was mainly concentrated in the destabilization failure stage. The initiation pressure of was 65%～70% of that of hydraulic fracturing. Compared with sandy mudstone,coal did not produce complex-through cracks for its development of stratification and joint weakness,large Biot coefficient,low elastic modulus and small brittleness coefficient.