Distributed Fiber-Optic Strain Monitoring of Fracture Propagation and Caging in Block Granite Samples

Fracture caging is a methodology which reduces stimulated volume of a reservoir by removing fluid within a fracture. Caged fractures are proposed to limit the stimulated volume of rock within Enhanced Geothermal Systems (EGS) and control induced seismicity. Mechanical observations of a caged fracture in granite have not been made despite successful flow experiments and models. Laboratory-scale hydraulic stimulation experiments are conducted in block-scale granite specimens under variable stress conditions. Distributed strain sensing (DSS) and acoustic emission (AE) monitoring are used to mechanically observe fracture evolution and to evaluate the effectiveness of fracture caging as a method for controlling fracture extent. Field-scale strain rate interpretation frameworks are applied with high-resolution DSS measurements. Strain and strain rate measurements align with AE results, while fracture propagation is slowed by intersecting wells. Hydromechanical processes underlying caged fractures in confined and unconfined conditions in granite samples are documented and described.