Effects of post-instability induced by supercritical CO2 phase change on fracture dynamic propagation

The effects of non-aqueous fracturing on unconventional reservoir stimulation have been investigated by laboratory and field work. Supercritical carbon dioxide (SC-CO2) is a notable non-aqueous fracturing fluid due to its potential to increase production, to reduce water requirements, and to minimize environmental impacts. In this paper, laboratory experiments conducted on polymethyl methacrylate (PMMA) are used to study the post-instability induced by SC-CO2 phase change on fracture dynamic propagation. Experimental results show that the fracture propagating speed of SC-CO2 fracturing is two order greater than that of hydraulic fracturing, and extensive hackle fractures occur in SC-CO2 fracturing. SC-CO2 phase change are mainly responsible for fracture post-instability, which dominates fracture propagation in SC-CO2 fracturing. Meanwhile, an equilibrium energy model of fracture dynamic propagation is proposed to describe the energy variation and transformation in porous medium. The concept of fracture propagation coefficient, i.e., the ratio of energy supplied by injection to fracture energy, is defined. SC-CO2 fracture propagation coefficient is much larger compared to hydraulic fracturing, suggesting that enough kinetic energy can accelerate fracture growth. This research may contribute to field application that SC-CO2 fracturing is likely to be an effective stimulation treatment, even in reservoir of undeveloped natural fractures.