Experiment investigation on the fracture initiation characteristics of shale saturated with CO2 and brine

Supercritical CO2 (SC-CO2) is regarded as a promising fracturing medium to exploit unconventional oil and gas resources. However, the reactions between CO2–brine–rock would change the mechanical properties and microstructure of rocks during SC-CO2 fracturing. To explore how the reactions affect the hydraulic fracture initiation and propagation, a series of fracturing experiments on shale saturated with CO2 or/and brine are carried out in this study. Experimental results show that the initiation pressures of rocks was reduced by 11.2 MPa after CO2 and brine saturation. Compared with dry shale and samples saturated with brine only, the initiation pressures of specimens after SC-CO2 and brine saturation reduce more significantly owing to the deterioration of mechanical properties by SC-CO2. And the fracture pattern of shale after treatment is more complex. Both tension and shear fractures are present and the fractures intersect and deflect.Moreover, fractures induced by the SC-CO2 fracturing of the untreated shale are more complex and rough than those in saturated brine and CO2. The arithmetic mean height, root-mean-square height, the maximum height and the fractal dimension of the crack surface of shale saturated with brine and CO2 decreased by 59.2%, 57.6%, 52.5% and 7.8%, respectively. The experimental results can provide laboratory evidence for the implementation of well-soaking before SC-CO2 fracturing.