DETERMINATION OF FRACTURE STORATIVITY IN HARD ROCKS USING HIGH-PRESSURE INJECTION TESTING

The determination of the storage capacity of fractures in crystalline rocks by means of hydraulic injection tests is studied by coupled hydr omechanical finite element simulations. The results verify that the st orage is related to the fracture opening, which is dependent on the co mbined stiffness of the fracture and the ambient rock mass. In most pr actical cases the storage is entirely controlled by the normal stiffne ss of the fractures. The strong coupling to the fracture opening impli es that the storage capacity can be estimated from the pressure depend ency of the fracture aperture in a high-pressure injection test. Such high-pressure injection tests can be conducted in addition to a conven tional low-pressure test to independently determine the storativity of the fracture. This provides an additional validation of the evaluated storativity, which implies not only that the value is more accurately assessed but also that other hydraulic properties can be determined m ore unambiguously. The method of high-pressure injection testing is ap plied in field experiments to deep fractures in granitic rocks at two sites,and its usefulness is demonstrated in an analysis of the field d ata.