Three-dimensional buoyant hydraulic fractures: finite volume release

Abstract

In impermeable media, a hydraulic fracture can continue to expand even without additional fluid injection if its volume exceeds the limiting volume of a hydrostatically loaded radial fracture. This limit depends on the mechanical properties of the surrounding solid and the density contrast between the fluid and the solid. Self-sustained fracture growth is characterized by two dimensionless numbers. The first parameter is a buoyancy factor that compares the total released volume to the limiting volume to determine whether buoyant growth occurs. The second parameter is the dimensionless viscosity of a radial fracture at the time when buoyant effects become of order 1. This dimensionless viscosity notably depends on the rate at which the fluid volume is released, indicating that both the total volume and release history impact self-sustained buoyant growth. Six well-defined propagation histories can be identified based on these two dimensionless numbers. Their growth evolves between distinct limiting regimes of radial and buoyant propagation, resulting in different fracture shapes. We can identify two growth rates depending on the dominant energy dissipation mechanism (viscous flow vs fracture creation) in the fracture head. For finite values of material toughness, the toughness-dominated limit represents a late-time solution for all fractures in growth rate and head shape (possibly reached only at a very late time). The viscosity-dominated limit can appear at intermediate times. Our three-dimensional simulations confirm the predicted scalings and highlight the importance of considering the entire propagation and release history for accurate analysis of buoyant hydraulic fractures.