A damage mechanics approach to the simulation of hydraulic fracturing/shearing around a geothermal injection well

Abstract Enhanced geothermal energy production requires the stimulation of natural fracture pathways to increase fluid flow within a reservoir while still effectively recovering the heat. During injection/production, reservoir permeability exhibits various degrees of enhancement or degradation with time. These changes are generally attributed to various multiphysics processes that act both during short-term stimulation and during production over the longer term. Important mechanisms of stimulation include tensile failure by hydraulic fracturing or shear failure by hydraulic shearing. A wide range of methods have been used to numerically simulate permeability enhancement in porous and fractured media including models based on damage mechanics, discrete fracture mechanics, critical shear strain criteria, effective stress, and even empirical permeability multipliers. We explore the use of damage mechanics to represent hydraulic fracturing/shearing within the reservoir. The model incorporates an energy release rate microcrack model in mixed modes (opening – I and shear – II) to simulate damage and permeability enhancement. The model is calibrated against compression tests to determine interrelationships between damage and both deformation and permeability. It is then applied to contrast both isothermal and thermal quenching effects during stimulation of hot reservoirs with cold fluid injection. The results illustrate that when fluid pressures are sub-failure, the damage zone is limited to the near wellbore region. As fluid pressure is increased, near wellbore mode II failure transitions to mode I hydraulic fracturing and rapidly increasing damage. A method of simulating cold water injection induced damage due to both shear and tensile failures is needed in the geothermal industry. This work offers a step forward in that direction.