Experimental study on the supercritical CO2 fracturing of shale

Supercritical CO2 fracturing technology,as an environmental friendly non-aqueous fracturing technology for shale gas extraction,has attracted more and more attention in the commercial exploitation of shale gas and shale oil. But the mechanism of crack propagation and the extension of supercritical CO2 in shale are still not well understood. In this paper,the experimental study of supercritical CO2 fracturing in shale was carried out by using the self-developed instrument for simulating the supercritical CO2 fracturing and the increase of shale permeability. The acoustic emission monitoring technology,combined with CT scanning technology,was employed to investigate the crack propagation law of the fracturing process,and the comparison of the permeability of shale before and after fracturing was conducted to evaluate the fracturing efficiency. The main conclusions are as follows:the fracturing and crack-propagation process can be divided into three stages,such as cavity filling stage,pore pressure accumulation stage and fracture propagation stage. The maximum counts of the acoustic emission is reached at the initiation pressure point,and the phase change occurs during the supercritical CO2 fracturing shale process;after fracturing,the permeability of the shale increases 3-4 orders of magnitude compared to that before fracturing,so the permeability increase is noticeable by fracturing;CT images show that the whole direction of the crack propagation tends to be parallel to the direction of the maximum principal stress,and the cracks are firstly initiated in the weak planes of wellbore,supercritical CO2 fracturing can in- duce complex fracture networks. This study can provide a scientific basis for the selection of the engineering parameters of the supercritical CO2 fracturing for shale gas exploitation.