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SELFRAC Report Summary

Project ID: FIKW-CT-2001-00182
Funded under: FP5-EAECTP C
Country: Belgium

EDZ and self-healing along a borehole - In-situ test at Mol (HADES)

In-situ test 4 is quite similar to in-situ test 2 but for Boom Clay. The natural closure of a borehole is studied. Permeability and possibly seismic measurements were performed to follow the evolution of flow properties through the borehole.

The testing is performed in the following way:
- Drilling of a test borehole
- Installation of the test equipment
- Monitoring of the borehole convergence (creep) and of the EDZ
- Hydro testing to evaluate transmissivity along borehole

We can conclude from in-situ test 4 that long-term seismic and AE measurements are able to reveal disturbances and detect the evolution of sealing/healing processes with time in Boom Clay by means of measuring the associated variations in seismic velocities, damping of amplitudes and frequencies of the seismic signals. Collapsing of an uncased borehole entails a decompression of the surrounding clay, which leads to an increase of deviatoric stresses and a decrease of pore water pressure. Both effects can lead to a decrease of seismic velocities and amplitudes, which was observed in the present seismic measurements.

After the decompression phase, the stresses as well as the pore pressure tend to recover. Due to on-going reconsolidation, the acoustic transmissibility of seismic waves increases again in Boom Clay. The increasing trend could be observed rather in the seismic amplitudes than seismic velocities. Experiments on a reinstalled fractured clay core demonstrated that seismic parameters recover and fractures seal within several weeks or months.

Reported by

2400 Mol
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