Determination of fracture storativity in hard rocks using high-pressure injection testing

Abstract

The determination of the storage capacity of fractures in crystalline rocks by means of hydraulic injection tests is studied by coupled hydromechanical finite element simulations. The results verify that the storage is related to the fracture opening, which is dependent on the combined stiffness of the fracture and the ambient rock mass. In most practical cases the storage is entirely controlled by the normal stiffness of the fractures. The strong coupling to the fracture opening implies that the storage capacity can be estimated from the pressure dependency of the fracture aperture in a high-pressure injection test. Such high-pressure injection tests can be conducted in addition to a conventional 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 more unambiguously. The method of high-pressure injection testing is applied in field experiments to deep fractures in granitic rocks at two sites, and its usefulness is demonstrated in an analysis of the field data.