Experimental Study on Injection-induced Fracture Propagation and Coalescence for EGS Stimulation

Abstract

Shear slip has been recognized as a dominant mechanism of permeability enhancement during the hydraulic stimulation in engineered geothermal systems (EGS). The so-called “hydroshearing” process usually occurs at treatment pressures below the minimum principal stress, and reactivates pre-existing fractures causing them to slip and dilate and possibly propagate in the shear and tensile modes creating new cracks. These processes can result in generation of a network with increased permeability for economic flow rates. We have performed injection experiments to investigate the possibility of flow rate increase through the propagation and coalescence of pre-existing fractures in granites under triaxial conditions. Results show that the pre-existing fractures can be propagated and coalesce at treatment pressures lower than the minimum principal stress during water injection. Also significant flow rate increase can be achieved through the fracture network generation caused by pre-existing fractures propagation and coalescence. In addition, both tensile wing cracks, shear and/or mixed-mode propagated secondary cracks can form. These observations show that the so called “hydroshearing” involves both shear slip and fracture propagation and contribute to the shear stimulation in EGS.