Successfully creating permeability in deep-seated rock masses to economically tap the heat to generate electricity remains a challenge for society. If solution can be found to unlock these resources a large amount of clean, local and renewable energy can be produced. Indeed, the Swiss Energy Strategy 2050 (ES2050) propose scenarios with 7% of national electricity supply from Deep Geothermal Energy (DGE), which corresponds to over 500 MWel installed capacity. In Switzerland, temperatures between 170-190°C are found at 4-6 km depths, but the natural permeability of deep crystalline basement rocks is too low to allow large volume fluid circulation for sufficient heat extraction. Therefore, permeability must be enhanced using high-pressure fluid injection to exploit DGE in an Enhanced Geothermal System (EGS).Two different paradigms are commonly referred to when discussing permeability creation processes through hydraulic injections: 1) hydraulic fracturing as the initiation and propagation of mode I fractures and 2) hydraulic stimulation, i.e. the mobilization of existing discontinuities in shear with associated dilation leading to a self-propping mechanism. The former is the common concept used in reservoir enhancement in the oil and gas industry, while the latter is typically the case in enhanced geothermal reservoirs. If it is currently accepted that both mechanisms can occur concomitantly, it is not well-understood how these processes interact and what rock mass characteristics and injection metrics controls which mechanisms dominate.To address these questions, relevant datasets from well-controlled hydraulic injection experiments are required. We propose to perform such experiments in an underground laboratory where rock mass conditions are representative of target formations for EGS in Switzerland, and with excellent access to the rock mass. Experimental work on small samples in loading frames suffers issues of scale because the samples tested are often homogenous and therefore unrealistic at larger scales. Well-controlled experiments at larger scales are often impeded by insufficient access to the rock mass. Funding to execute a well-controlled experiment at the Grimsel test site has secured under the umbrella of the Swiss Competence Center for Energy Research - Supply of Electricity (SCCER-SoE), and an experimental plan has been developed. This experiment is referred as the In-situ Stimulation and Circulation experiment (ISC-experiment). The ISC experimental plan focusses on the stimulation of existing shear zones, and includes an extensive pre- and post- stimulation rock mass characterization program and a comprehensive monitoring package. This unique setting will allow observations and measurements in a coupled manner of all parameters relevant to geomechanics (i.e. stress state, deformation, fracturing, pressure propagation during injection and following shut-in, etc.), hydrogeology (i.e. fluid flow, permeability, fracture connectivity, etc.) and seismology (i.e. a micro-seismic monitoring system at multiple scales).The objective of this proposal is to expand the ISC experimental plan by adding a research component dedicated to understanding and modeling the initiation and propagation of hydraulic fractures. This research will address unresolved questions regarding the initiation, propagation, and interaction of hydraulic fracturing in tough crystalline rocks. Multiple hydraulic fractures will be created using a variety of injection strategies to determine their behavior and compare their impact. The characterization and monitoring systems in place will allow precise mapping of hydraulic fracture evolution, and their impact on the pore pressure field and the rock mass. Essential parameters controlling hydraulics, such as fracture aperture changes during injection, will be measured. The seismic response to hydraulic fracturing will be measured and compared with shear zone stimulation. The research plan is composed of three main components: 1) execution of the hydraulic fracturing tests, 2) characterization of the fractured volume and the analyses of the collected data, and 3) benchmarking of hydraulic fracturing simulation tools against the field data set. Results of this research will significantly contribute to the understanding of injection strategies to truly engineer permeability creation in the rock mass at depth for deep geothermal systems.