Laboratory hydraulic fracturing experiments on crystalline rock for geothermal purposes

This article reviews laboratory experimental studies on hydraulic fracturing under triaxial and true triaxial stress conditions in crystalline rock for geothermal purposes, and places particular focus on the stimulation of Enhanced Geothermal Systems. First, parameters that influence hydraulic fracture initiation and propagation and breakdown pressure are reviewed and discussed. The parameters including micro-structure, fluid viscosity, injection rate, and fluid infiltration, and stress conditions are identified as the key controlling factors in hydraulic fracture growth in hard rock. Second, innovative injection schemes, such as cyclic and fatigue hydraulic fracturing, are reviewed because they show advantages both in fracture network creation in granite and in mitigating and controlling induced seismicity via fluid injection. Third, this review includes fracture-inspection techniques, non-destructive methods of acoustic emission (AE) monitoring and X-ray computed tomography (CT), and microscopic observations used for quantifying the efficiency of injection protocols. In addition to AE parameters, such as AE event rate and source location, we emphasize the importance of in-depth AE analysis on the failure mode and radiated seismic energy. X-ray CT and microscopic observation enable fractures in the rock volume to be quantified, and thereby lead to a better understanding the mechanism behind hydraulic fracturing. Combined measurements of AE and CT yield insights into the complex process of hydraulic fracture and permeability enhancement. The discussion section is enriched with diagrams that connect the injection rate and the resulting fluid infiltration zone and fracture process zone, granite-specific hydraulic fracturing behavior, and practical upscaling elements for potential field applications in geothermal fields.