Phase-Field Modeling of Fracture in CO₂ Sequestration

This work studies fracture propagation in the encasements of wellbores, and how extreme pore pressures and pressure rates affect their damage states. The initiation and evolution of fractures in poroelastic media often gives rise to discontinuous fields within computational problems. Such models present computational challenges due to the complexity of sharp discontinuities that arise during the finite element solution and also lack the ability to model fracture initiation. These challenges may be alleviated by using a phase field formulation of fracture mechanics, which introduces a continuous, diffusive scalar damage field around crack surfaces. We make use of a direct computation of the crack’s width, or joint opening, by using the gradient of the phase field in the damaged area. This is useful for determining the material’s fluid-mechanical properties, such as the calculation of Poiseuille-type flow that occurs within a sufficiently damaged medium. Using this approach, it can be shown that the evolution of such fractures in a porous medium can contribute to the material’s permeability, hence coupling fluid flow and damage within a material. Conversely, the movement of fluids through a damaged solid can influence the material’s fracture distribution driven by pore pressures, meaning that there is a two-way coupling of damage and fluid flow. Using the Sierra Mechanics code suite at Sandia National Laboratories, a phase-field model of fracture is developed which will allow a loose, two-way coupling of these physics for future implementations. We find the model’s ability to predict fractures initiated and propagated by increasing pore pressure to be consistent with fractures that occur from fluid injections. We also find that the joint openings calculated with this model will be helpful for implementing Poiseuille flow along fractures.

The coupled physics of pressurized fluids in porous rock and fracture propagation is essential in subsurface rock mechanics. Applications in fields such as hydrology deal with the distributions of contaminants in groundwater, which are often dependent on fracture distributions in the earth’s crust, and the phenomenon of fluid induced fault activation is of great interest in the field of geophysics. The concept of carbon sequestration is contingent upon pressurized fluids, and CO₂, being contained within poroelastic substrates of the subsurface. In

, it is shown that under certain pressure conditions, CO₂ and brine will flow along existing joints and faults when an injection occurs at depth. This implies that the CO₂ intended to be sequestered may leak over time.