Experimental study on fracture initiation pressure and morphology in shale using supercritical CO2 fracturing

The clay content of shale gas reservoirs in China is high,and most shale reservoirs are located in water-defi- cient areas. The SC-CO2 fracturing is expected to be a new non-aqueous fracturing method because of its characteris- tics of low viscosity and low surface tension. In this paper,the comparison tests of SC -CO2 fracturing and hydraulic fracturing in shales and sandstones were carried out under triaxial conditions. The fracture morphology was investigated by CT scanning and the effect of temperature on fracturing was studied. The results show that the initiation pressure of SC-CO2 fracturing in shale is about 50. 9% lower than that of hydraulic fracturing,and the initiation pressure of SC-CO2 fracturing in sand is about 57. 1% lower than that of hydraulic fracturing. Compared to hydraulic fracturing,the pressurization rate of SC-CO2 fracturing is slow due to the compressibility of CO2 . Furthermore,the SC-CO2 fracturing in shale has repeated cracking phenomenon,and it is easier to induce complex cracks. With the increase of tempera- ture,the initiation pressure by SC-CO2 fracturing decreases. The CT scanning shows that SC-CO2 fracturing in shale is easier to form multiple network cracks and achieves the similar effect of stimulated reservoir volume.