Anisotropic damage evolution around boreholes in chemically reactive formations

Securing a sufficient energy supply to match global demand is a key challenge of our modern society. Currently, the vast majority of energy resources is extracted from the subsurface; in the future, new resources - such as unconventional fossil and geothermal energy - will constitute a significant portion of the energy portfolio. This thesis is motivated by the challenges that stem from harvesting the energy of unconventional fossil and geothermal reservoirs. Both unconventional fossil and geothermal energy developments are pushing into new domains, increasing the technological risks. Dominant risk factors associated with subsurface energy resources are borehole drilling operations and the subsequent stimulation of the reservoir. This thesis develops a new science approach for borehole geomechanics, considering (in addition to classical mechanics) the constraints from non-equilibrium thermodynamics. Constitutive models covering anisotropy, damage evolution and chemical weakening are derived from non-equilibrium thermodynamics and are combined with advanced numerical methods. The overarching hypothesis is that this combined approach offers a new way to address problems in borehole geomechanics. The thesis is organized in a series of papers showing that: (a) coupled processes lead to self-localization phenomena not represented in the classical approach, (b) in the classical approach anisotropic elasticity can be addressed but not anisotropic failure, (c) the new approach is successful in modeling borehole problems with progressive anisotropic failure (anisotropic damage evolution) and with time dependent chemical weakening. The main nding is that the new approach can describe irreversible processes, such as failure and chemical weakening, because it considers the underpinning dissipative mechanisms explicitly.