In this paper we present results of fluid injection and compression tests under triaxial and true triaxial stress states representing the current knowledge of the stress regime at the EGS Collab site. The fracture properties and failure mode of rocks are known to depend strongly on the rock type and the stress state–understanding of which is critical to development of geothermal stimulation techniques involving hydraulic fracturing and shear stimulation. Collab core specimens are studied to better understand the rock and natural facture response to stress and pore pressure increases via injection. Natural fracture stiffness and permeability variation due to slip are measured, and the response of the rock to wellbore injection is characterized. Acoustic emission response during testing is also measured and studied in conjunction with stress/deformation data to better understand the evolution of the failure plane and the associated micro-cracking pattern. CT-scan images of the failed specimens are used to study the failure evolution process and the role of rock fabric features such as foliation and healed fractures.