Phase transition and thermal dynamics in response to fracture propagation in CO2 fracturing

The quantification of thermo-fluid response during fracture propagation remains an outstanding challenge in CO 2 fracturing, limiting the understanding of complex fracture networks generation mechanism. In this paper, an unsteady flow and heat transfer model in Lagrangian framework is developed, which can simulate the phase transition and thermal dynamics of CO 2 induced by transient pressure step during fracture propagation. The results show that drastic volume expansion is induced by the transient pressure step at early propagation stage, generating a distinct gas-liquid phase zone adjacent to the fracture interface and a substantial temperature gradient along the fracture length (up to 202 K within 0.54 m). Global observations reveal a progressive temperature increase over time, primarily attributed to CO 2 compression and kinetic-to-internal energy conversion at the fracture toe. The CO 2 phase transition dynamics are controlled by two key parameters: fracture-tip length and initial fracture-tip pressure. The two-phase zone emerges only when the initial fracture-tip pressure is below its threshold (3.5 MPa here). And its persistence duration and maximum width positively correlated with fracture-tip length. Furthermore, fracture propagation is primarily driven by fluid accumulation and expansion cooling effect. The model can provide more fundamental insights for thermodynamic behavior in CO 2 fracturing. • An unsteady thermo-fluid model is developed for fracture propagation. • The local phase transition and thermal effect of CO 2 caused by pressure step can be quantized. • Gas-liquid phase zone emerges with an initial fracture-tip pressure below 3.5 MPa. • Fracture propagation is primarily driven by fluid accumulation and expansion cooling effect. • CO 2 undergoes global compression with local expansion during fracturing propagation.