Phase-field modeling of CO2 fracturing: effects of natural fractures and formation interlayer contrast

CO₂ fracturing has been recognized for its significant advantages in enhancing petroleum recovery. In this study, we establish a finite volume method (FVM)-based phase-field model to simulate CO₂ fracturing in poroelastic media, accounting for fluid property variations with temperature and pressure. The porous medium is modeled using classical Biot poroelasticity theory, while fracture behavior is described through a phase-field framework. The phase field acts as an interpolation function to smoothly transition properties between the fracture and reservoir regions. The governing equations are discretized using an implicit cell-centered FVM and solved via an iterative staggered scheme. CO₂ thermophysical properties are updated via the Peng–Robinson equation of state and the Lohrenz–Bray–Clark viscosity model. The framework is verified through a 2D notched specimen subjected to injection of water and CO₂. It is then applied to investigate fracture propagation in both homogeneous and heterogeneous shale reservoirs. For the homogeneous cases, we analyze the effects of horizontal principal stress differences and natural fracture approach angles. For the heterogeneous reservoirs, we examine how natural fracture orientation and interlayer elastic modulus contrast affect fracture morphology and propagation. This study demonstrates the robustness of the proposed framework in capturing complex fracture behavior, offering insights into optimizing CO₂ fracturing for unconventional gas and oil extraction.