Quantifying the Porosity of Crystalline Rocks by In Situ and Laboratory Injection Methods

Andreas Möri,Marja Siitari-Kauppi,Markus Leuenberger,Kunio Ota,Florian Eichinger,Martin Mazurek

doi:10.3390/min11101072

Abstract

The porosity and pore geometry of rock samples from a coherent granodioritic rock body at the Grimsel Test Site in Switzerland was characterised by different methods using injection techniques. Results from in situ and laboratory techniques are compared by applying innovative in situ resin impregnation techniques as well as rock impregnation and mercury injection under laboratory conditions. In situ resin impregnation of the rock matrix shows an interconnected pore network throughout the rock body, consisting mainly of grain-boundary pores and solution pores in magmatic feldspar, providing an important reservoir for pore water and solutes, accessible by diffusion. Porosity and pore connectivity do not vary as a function of distance to brittle shear zones. In situ porosity was found to be about 0.3 vol.%, which is about half the porosity value that was determined based on rock samples in the laboratory. Samples that were dried and impregnated in the laboratory were affected by artefacts created since core recovery, and thus showed higher porosity values than samples impregnated under in situ conditions. The extrapolation of laboratory measurements to in situ conditions requires great care and may not be feasible in all cases.

Highlights

Matrix pore space, typically water-saturated under in situ conditions, represents a large water reservoir even in low-porosity crystalline rocks and plays an important role in solute transport and retention over long times
On-site work was performed at the Grimsel Test Site (GTS), the underground rock laboratory of Nagra (National Cooperative for the Disposal of Radioactive Waste, Switzerland) in the Swiss Alps, where various experiments aiming at the characterisation of crystalline rock are carried out (e.g., [11,12])
The present study provides porosity values in crystalline rock pertinent to in situ conditions, obtained from a coherent granodioritic rock body at the Grimsel Test Site (GTS)

Summary

Introduction

Typically water-saturated under in situ conditions, represents a large water reservoir even in low-porosity crystalline rocks and plays an important role in solute transport and retention over long times (see for example [1,2,3]). In many concepts quantifying mass transfer in fractured media, advective-dispersive transport is thought to be limited to fractures, whereas transport into/out of the adjacent microporous matrix is dominated by diffusion [4,5,6] In this dual-porosity concept, considered in several programmes for radioactive waste disposal, matrix porosity offers a large storage volume for radionuclides (e.g., [7,8,9,10]). It follows that a good understanding of in situ matrix porosity, its connectivity and spatial variability is required to quantify the degree to which matrix diffusion affects contaminant transport in crystalline rocks. On-site work was performed at the Grimsel Test Site (GTS), the underground rock laboratory of Nagra (National Cooperative for the Disposal of Radioactive Waste, Switzerland) in the Swiss Alps, where various experiments aiming at the characterisation of crystalline rock are carried out (e.g., [11,12])

Objectives

Methods

Results

Discussion

Conclusion