Coupled Processes in Fracture Propagation in Geo-Materials: from Hydraulic Fractures to Earthquakes
June 10, 2019 — June 14, 2019
- Harsha Bhat (Ecole Normale Superieure, Paris, France)
- Brice Lecampion (Ecole Polytechnique Fédérale de Lausanne, Switzerland)
The propagation of fractures in geological media either due to natural or man-made forcing share common similarities related to the coupling of fluid and temperature variations with mechanical deformation. Earthquakes are shear mode cracks dominated by frictional weakening with thermal pressurization, flash heating of asperities playing an important role. Hydraulic fractures on the other hand are opening mode cracks propagating under the injection of viscous fluid. In both cases, the energy budget of the process (and the overall evolution of the system) may widely differ depending on the dominant physical process: e.g. fracture surface creation versus viscous flow dissipation, fluid storage versus leak-off in hydraulic fractures.
During this summer school, we will focus on recent advances in the mechanical modeling of both fluid-driven fractures and earthquakes. Our aim is to review the physical modeling of these problems, which combines fracture mechanics, thermo-hydro-mechanical deformation and complex friction laws. We will cover in-depth the solution of a number of model problems that allow for a deeper understanding of the complex interplay between the different physical processes on the evolution of fracture.
The theory of fracture mechanics and the use of boundary integrals equations for the solution of fracture problems will be discussed in details in order to give a basis for its applications to both earthquakes nucleation and hydraulic fracture growth.
After a presentation of the coupling between fluid flow and fracture deformation, the tip behavior of a propagating hydraulic fracture will be discussed in details as well as semi-analytical solutions for the growth of finite hydraulic fracture of simple geometries (radial, plane-strain) in limiting propagation regimes (Storage/viscosity & toughness -- leak-off/viscosity & toughness). Insights revealed by dimensional analysis and scaling will be highlighted. Different numerical schemes for hydraulic fracture propagation will be discussed and a particular emphasis will be given to numerical scheme incorporating the multiscale tip behavior. Hands on examples with an open-source code will demonstrate a number of important points for the resolution of such moving boundary problem. The necessity of proper code verifications will be highlighted. A number of laboratory and field experiments on hydraulic fracture growth will be given and compared to theoretical predictions. Current research topics related to rock anisotropy, heterogeneity and non-linearity will also be discussed.
On the subject of earthquake source physics, the theory of fracture mechanics will be revisited in the context of Mode II and Mode III fractures. Insights into solutions for cohesive zone fractures will be given along with the reasoning for their applicability in earthquake source physics. Both analytical and numerical treatment through Boundary Integral Equation Methods will be given. The students will then be given insights into these fracture solutions through laboratory experiments. At the next stage we will talk about elaborate friction laws used in earthquake physics and how they are coupled to classical fracture mechanics problems. Insights from laboratory experiments on friction and dynamic shear rupture will also be given. As a final step a short introduction to field geology would be given where these modeled phenomena can be observed.