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POSIVA Report 2019-3

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Name:

HYDCO Summary Report

Writer:

Öhman, J., Hansson, K., Andersson, P., Joutsen, A., Poteri, A., Ahokas, H., Heikkinen, E.

Language:

English

Page count:

153

ISBN:

978-951-652-273-2

Summary:

HYDCO (acronym for HYDraulic COnnectivity) is a project intended to study the hydraulic connectivity in a rock volume that is representative of the host rock for the planned repository for spent nuclear fuel at the Olkiluoto site. The investigations were carried out from an investigation niche of the ONKALO underground facility (Posiva, 2003), at a depth of ca 350 m below ground surface. The studied rock volume was characterised by a multi-disciplinary investigation in two approximately 25-m long drillholes, which are sub-parallel with a separation distance of two meters. The data from the three disciplines geology, geophysics, and hydrogeology are co-interpreted to form a hydrogeological characterisation of the studied rock volume. The objectives of this study are to analyse: 1) fracture geometry, 2) hydraulic cross-hole connectivity, and 3) the existence of so-called compartmentalised open connected fractures. In this poorly conductive setting, the fracture properties do not necessarily stand out as distinct contrasts against what is normally perceived as the background heterogeneity of non-fractured rock. This poses a challenge in field investigations and sets a limit for the characterisation of fracture flow and the hydraulic connection between the drillholes. Consequently, a secondary goal of the HYDCO project is to evaluate different investigation methods in terms of their ability to contribute to the hydrogeological characterisation in poorly water conductive rock mass.

The lithology at the HYDCO niche is dominated by veined gneiss (VGN) and coarse-grained granitic pegmatoid (PGR). The fracture frequency seems to correlate with lithology, with higher fracturing in the VGN. The fractures mapped in the HYDCO drillholes are divided into three sets. The strongest set, in terms of intensity, is N-S striking and almost vertical. The second set is gently dipping and sub-parallel to the foliation of gneiss. This second set is suspected to be a local set related to the foliation, as it is comparatively rare in the surrounding tunnel data. However, the orientation of the second set is common in the Olkiluoto bedrock but it is locally lacking at the HYDCO site. It is noted that this second set is primarily associated to slickensided fractures, and owing to its orthogonality versus the drillholes, it has a key role in crosshole fracture connectivity. Nearby tunnel-mapping data indicate the presence of a potential third set with unfavourable orientation for drillhole intersection.

As the starting point for the multidisciplinary co-interpretation, potential cross-hole structures are first identified based on geological/geometrical observations. This analysis is based on pair-wise fracture intercepts with concurring orientations and similar geological properties. Out of these, one of the most prominent structures is associated to a fractured zone mapped in the core of one drillhole and a Tunnel-Cutting Fracture (TCF) in the ONKALO tunnel (ca 20 m in length). The fractures in the fracture zone belong to sets 2 and 3.

Then, the identified potential cross-hole structures are used as a framework for interpreting the following investigations: 1) Mise-à-la-Masse connections, 2) cross-hole coupled radar reflectors, 3) detailed PFL flow logging, and 4) a set of detailed hydraulic interference tests using multi-packer systems. As a preparatory step to structural inference of the detailed interference test, the array of observed hydraulic responses were first delineated in terms of so-called conceptual hydraulic connections (i.e. perceived as “the common denominators” in the response pattern). In a second step, these hydraulic connections are associated to geological structures (i.e. cross-hole structures, PFL anomalies, and fractures). The outcome is a hydrogeological characterisation of the key hydraulic connections in the HYDCO rock mass. The characterisation is used to evaluate the conformity and significance of the contribution from the various investigation disciplines.

The HYDCO rock-mass characterisation has provided practical experience and improved understanding of the conditions that can be expected around deposition holes, which is valuable as the use of investigation drillholes will be limited in the future repository. This study has demonstrated that most of the applied investigation methods are useful for identifying fracture connections in the studied rock mass. However, in many cases the fracture properties are close to the detection limit, which demonstrates a need to define cut-off points (i.e. a limit below which data are negligible and can be lumped as “minute heterogeneity”). The evaluated transmissivities, ranging from discrete PFL anomalies to meter-scale injection tests, typically fall below 10‑10 m2/s; hence, the studied rock volume does not support the existence of “compartmentalised open connected fractures”, i.e. irrespectively of isolation, the fractures cannot be referred to as “transmissive”).

However, the lack of observations does not necessarily guarantee the absence of this type of fractures in the studied volume (i.e. the drillholes provide only partial coverage of the studied volume; if fracture flow exhibits within-plane channelling, or if fracture geometry is of unfavourable orientation, the probability of intersection may be small). Furthermore, the degree – to which the outcome of this local-scale investigation can be generalised for the entire repository volume – is questionable.

This study has also demonstrated the difficulty in resolving a three-dimensional depiction of the fracture-network connectivity between the two drillholes. Therefore, it appears that a minimum of three drillholes is required to improve the characterisation of the fracture network. The key components in the modelling of cross-hole connections are: 1) detailed geological core logging, 2) high-resolution drillhole imagery (ABI/OBI), possibly supported by drillhole radar, 3) PFL logging and 4) hydraulic injection tests with monitoring of hydraulic responses.

Keywords:

Groundwater, flow, hydrogeology, bedrock, connectivity, interpretation, investigation, ONKALO, ONK-PP262, ONK-PP274, hydraulic interference, Posiva Flow Log

File(s):

POSIVA 2019-03_web (pdf) (13.4 MB)


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