Abstract

Local hydrocarbon occurrence in tight salt rock has been intensively investigated during the site exploration for the disposal of high-level radioactive waste. Special in-situ measurement equipment has been engineered to quantify the amount of gaseous and liquid hydrocarbons and to monitor pressure development with high resolution. Based on the data analysis it can be concluded that such kind of occurrences are bounded locally in the tight rock and is only then mobile when local channeling with high permeability is created due to stress redistribution by drilling or excavation.

1 Introduction

Fluids including both liquid and gas phases can be found within the crystal structure or along grain boundaries in all types of sedimentary rock and were formed by accumulating and reacting of different mineral and/or organic particles under pressure and temperature conditions during the genesis. These small inclusions range in size of several micrometers and are usually invisible in detail without microscopic studies. However, these fluids usually dispersed in a very low amount can form local accumulations in a rock volume up to some cubic metres. In an underground facility in salt rock, hydrocarbon occurrences have been found during the site exploration for the disposal of high-level radioactive waste in north Germany.

The initial hydrocarbons in undisturbed rock are situated under petrostatic pressure which is much higher than hydrostatic pressure in deep underground. Because of the very low rock permeability, migration of such fluids is almost impossible even under the high pressure-gradient condition. Fluid release from the crystal structure will take place if the stress state changes. In case of drilling or excavation, stress will be redistributed with the result of a deviatoric stress state. If fluid pressure is higher than the minimal stress, dilatancy-controlled fluid migration occurs (Xu et al. 2013). This results to the generation of micro-fissures between crystal structures with an increased permeability.

With regard to the long-term performance of a potential repository, it is important to characterise the distribution, amount and interconnectivity of the fluid inclusions. It is also important to determine the permeability of micro-fissures and to characterise the hydraulic properties after the fluid release.

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