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Hydrological Characterisation of Fractured Rock (Sellafield)

Objective

The overall objective of the project is to decrease uncertainties in the hydrogeological and structural characterisation of a volume of rock at Sellafield in Cumbria. This rock is currently being considered as a potential host for a low and intermediate level solid radioactive waste repository in the United Kingdom.
The specific aims of the work are to perform geophysical and hydrogeological tests in and between two boreholes in a fractured and layered volcanic rock sequence, the Borrowdale Volcanic Group.
Geophysical data will be integrated into a model of the rock volume. The results of single and crosshole hydrogeological testing will be used to clarify the lithological and structural features or combinations which control groundwater flow in the Borrowdale Volcanic Group.
This model will be applied to develop and validate concepts of groundwater flow in the immediate vicinity of the proposed repository.
Methodologies for cross borehole geophysical testing and their applications at Sellafield have been reviewed. Separate design intent memoranda for borehole radar and cross borehole seismics have been produced. Based on this work it was concluded that cross borehole radar would not be feasible at Sellafield, and that due to the saline groundwaters in the Borrowdale volcanic group (BVG) the anticipated penetration of single borehole radar was uncertain.

Wireline logs and vertical seismic profile (VSP) tests have been run in both boreholes 4 and 2 as a standard element of the sequence of drilling and logging exploratory boreholes at Sellafield. The suite includes: caliper log, dual lateralog, dipole shear sonic imager, litho density log, natural gamma ray spectroscopy log, compensated neutron log, formation microimager, acoustic borehole televiewer, geochemical logging tool and vertical seismic profiling. There is significant variability in the logs, representing the significant heterogeneity inherent in the Borrowdale volcanic group. A trial of omnidirectional single borehole radar and directional single borehole radar has been carried out in borehole 7A at Sellafield. The ABEM RAMAC borehole radar system was used. The performance of the radar trial in the BVG was poor. At resistivity levels of less than 100 Ohms, no direct radar signal was detected by the downhole receiver. Evidence from the formation microimager, sonic televiewer, core logs and single borehole packer tests confirmed that these areas of low resistivity corresponded to areas of major faulting, zones of fracturing and regions of high hydraulic conductivity.

However, the maximum predicted range was 35 m in an unfractured zone between 620 and 800 m depth in borehole 4. In the fractured zones, range fell to less than 10 m. It is concluded that continued radar reflection surveys in the BVG at Sellafield will not significantly enhance the data obtainable from the dual lateralogs and anticipated from t he cross hole seismics.

Profiles of hydraulic conductivity and points of inflow into borehole 2 showed no immediate apparent correlation between structure and the location of inflows into the borehole. A limited additional programme of longer term straddle packer tests using variable drawdowns increasing in 5 steps to 150 m drawdown were carried out, focusing on zones of inflow into borehole 2. In general, the distribution of hydraulic conductivities for bulk flow in the BVG centres in the range 1.0e-11 to 1.0e-9 m/s.

During the post drilling tests in borehole 2, the pressure in the full open hole section of borehole 4 was monitored. There was no evidence of hydraulicinterference at the low applied drawdowns between boreholes 2 and 4. Variations in the pressure monitored in borehole 4 were seen on a 12 and 24 hour basis, probably related to tidal effects. However, the amplitude of these cycles was close to the resolution of the gauges.
Work programme:

1. Review methodologies and prepare Design Intent Memorandum.
2. Geophysical testing in Sellafield borehole No. 4.
3. Cross-borehole geophysical testing.
4. Single borehole hydraulic measurements in borehole 4.
5. Design of cross-borehole interference testing.
6. Cross-borehole interference testing.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Nirex Ltd
Address
Curie Avenue Harwell
OX11 0RH Didcot
United Kingdom

Participants (1)

Sir Alexander Gibb and Partners Ltd
United Kingdom
Address
427 London Road
RG6 1BL Reading