Permeability Measurements of Fractured Granite at 350-450 °C Under Confining Stress

摘要

A new concept of enhanced geothermal system, in which a reservoir is created in ductile basement rocks, has been proposed recently. One of possible ways to create a fracture network in a ductile rock is thermal and/or hydraulic fracturing. Although creating fractures may be possible, there is concern about the permeability of the fractured rock after recovery of temperature and/or effective confining stress to the initial state, at which plastic deformation of fracture may occur. The present study has experimentally explored permeability of thermally fractured granite at various combinations of temperature (up to 450 °C) and effective confining stress (up to 90 MPa). It has been found that transition from elastic to plastic deformation occurs at a specific stress level (i.e., the elasticplastic transition stress), depending on temperature. The elastic-plastic transition stress decreases with increasing temperature, for which an empirical equation has been presented. At both elastic and plastic conditions, relations between log permeability and log effective confining stress are linear, and the slope of the linear curves are independent of temperature. Reduction of log permeability by increment of log effective confining stress is larger for the plastic condition. Based on these findings, a way to roughly predict permeability of fractured granite at various combinations of temperature and effective confining stress has been developed. It indicates that the method can provide reasonable prediction results at both elastic and plastic conditions.