Investigating the aperture of hydraulically conductive fractures in crystalline rock by electrical means.

Abstract:

In the siting of a deep geological repository for spent nuclear fuel, detailed characterisation of the groundwater flow pattern in the aquifer is essential. In Sweden the Forsmark site is planned to host the repository. This site features sparsely fractured crystalline rock of “granitic” type and groundwater flows in discrete fractures.

The host rock has been thoroughly examined and one of many aims of the hydrogeological investigations has been to obtain an empirical correlation between the fracture transport aperture e_t (m) and the transmissivity T (m²/s). The fracture transport aperture is a parameter that cannot be directly measured but only estimated, typically from hydrogeological investigation. Based on these, and on modelling studies, a best estimate correlation between the fracture transport aperture and transmissivity was assumed in the safety assessment SR-Site, where e_t = 0.5∙T^0.5.

The local aperture of discrete fractures that intersect a borehole can also be estimated from geophysical borehole loggings of the rock resistivity. Here an electrical current is emitted into the surroundings of the borehole. In non-fractured parts of the borehole, the obtained resistivity will represent the resistivity of the rock matrix. If placing the logging tool in front of a hydraulically conductive fracture, the obtained resistivity will party represent the rock matrix and partly represent the free groundwater in the fracture. By a number of geometrical considerations, and by knowing the electrical conductivity of the groundwater, the local fracture aperture can be obtained. It should be noted that this aperture is likely somewhat larger than the transport aperture, as the electrical current will be propagated in the entire water volume and is not subjected to drag from the fracture surfaces. The obtained aperture is, hence, called the volumetric aperture e_v (m).

160 flowing structures at the Forsmark site have been investigated in this manner, resulting in a cumulative distribution function (CDF) of the volumetric aperture. This is compared to the (modelled) CDF of the fracture transport aperture obtained by the e_t = 0.5∙T^0.5 correlation. As the transmissivity has been measured independently for many of the flowing structures, the volumetric aperture is also plotted versus the fracture transmissivity.