Dynamics of Natural Rock Fractures in the EGS Revealed Via the Pressurized Water Injection Experiments in Laboratory

Abstract

In order to suggest the optimal design of Enhance Geothermal Systems (EGS), we need to understand the dynamics of pre-existing rock fractures during pressurized water injection into the geothermal reservoirs precisely. Especially, the shear slip on fractures and the fracture propagation from tips/asperity contacts of fractures should be carefully investigated and modeled, since these mechanisms could vary the mechanical and hydraulic properties of geothermal reservoir drastically. Our study presents the experimental results those explore the linkage between strength, strain/slip distance, permeability, and acoustic emissions of the fractured rock of granite during hydraulic shearing/fracturing under high stress condition (e.g., both normal and shear stresses on pre-existing fracture are more than 50 MPa) assuming the geothermal reservoir environment. Through the experiments, we successfully constrain the failure criteria, maximum strain/slip distance, maximum permeability change, and so on for the fractured rock in laboratory-scale. Based on these experimental results, we further discuss how these parameters vary depending on the representative scales of reservoir. Such information would be indispensable to determine the actual injection protocol in the EGS developments.