Experimental study on the growth behavior of supercritical CO2-induced fractures in a layered tight sandstone formation

Abstract

As a non-aqueous fracturing technology, carbon dioxide (CO2) fracturing is eliciting increasing attention in the exploitation of tight oil/gas reservoirs. This study explored the growth behavior of supercritical CO2 (SC-CO2)-induced fractures in layered tight sandstones through a series of laboratory fracturing experiments under triaxial stress states. Acoustic emission (AE) monitoring and computerized tomography scanning were performed to analyze the AE focal mechanisms and fracture geometry. The features of SC-CO2 fracturing were discussed through a comparison with the features of x-linked guar and slickwater fracturing. The influences of horizontal differential stress and pumping rate during SC-CO2 fracturing were also examined. Results showed that SC-CO2 likely promoted the dilation or shearing of bedding planes and natural fractures, resulting in a complex fracture network even under a high horizontal differential stress. Statistical analysis of the P wave polarity of AE waveforms indicated that the AE events associated with the SC-CO2 induced factures were dominated by shear failure. Most of the branching fractures induced by SC-CO2 fracturing were extremely narrow that they impeded proppant transport. For SC-CO2 fracturing, injection pressure (especially breakdown pressure) was low and likely dissipated as the growth of branching fractures. AE activities, indicating the fracture growth, more likely occurred when the injection pressure rose high (e.g., in tens of seconds before and after breakdown) or fluctuated remarkably. A large pumping rate was necessary to maintain high injection pressure which can open the branching fractures widely and persistently because of the extremely large leak-off rate of SC-CO2.