UC Santa Barbara

Fluid-driven subsurface fractures interact with a variety of heterogeneous elements in the surrounding environment, including fluid-filled pores, material discontinuities, and other cracks and fissures. Their complex propagation is governed by fluid-solid interactions, characterized by nonlinear and coupled dynamics between the flow and the fracture. This thesis focuses on addressing a variety of such problems related to fluid-driven fractures or hydraulic fractures within increasingly realistic situations. Initially, we examine the post-shut-in behavior of a hydraulic fracture in the viscous regime, where viscous dissipation is the dominant form of energy dissipation. Subsequently, we investigate the propagation of a hydraulic fracture driven by displacement flows of two immiscible fluids and the propagation of a hydraulic fracture across a material discontinuity.

When pressurized fluid is injected in a homogeneous infinite solid medium, a simple penny-shaped fracture forms and grows in the direction of the minimum confining stress. This type of fracture offers a representative focus for experimental investigations in a laboratory environment, serving as a powerful tool to understand the various physical mechanisms governing the growth of the fracture. Throughout the thesis, we vary different material properties of the solid media and fracturing fluids, including the Young's modulus of the solids, the viscosity of the fluids and, the flow rate of the injection. Gelatin serves as the clear brittle elastic solid medium, that allows us to observe and record the fracture growth because of its transparent nature.

In the first study, we examine the post-shut-in behavior of a penny-shaped hydraulic fracture in the viscous regime. We measure both the fracture aperture and radius, noting that the fracture radius continues to grow slowly over time even after injection stops, until it reaches a saturation point.

Next, we investigate the injection of an immiscible fluid at the center of a liquid-filled fracture. We study the displacement of the interface between the two fluids and its effect on fracture propagation. We conduct experiments and derive scales to predict the growth dynamics of the fracture.

Finally, we present an experimental investigation into the propagation of hydraulic fractures in layered brittle media in the toughness regime, where the creation of new fracture surfaces is the dominant means of energy dissipation. We report that the relative stiffness of the initiating layer significantly influences fracture propagation: A fracture that forms in a soft layer remains trapped, whereas a fracture that originates in a stiffer layer experiences a rapid fluid transfer into the neighboring softer layer upon reaching the interface. Additionally, we present a quantitative model that captures the competing effects of elastic deformation and fracture propagation and report good agreement with experiments.