Overcoring procedure

In late 2010, the pre-grouted borehole LCS06-001 (link to grout pre-test) was overcored to obtain a detailed picture of fracture system and where the injected grout has penetrated to. Additionally, the opportunity was used to further develop the overcoring technique to be used for stabilising and sampling currently running experiments on fractures.

The following steps summarize the overcoring procedure (Fig. 1):

1. Drilling of the 300 mm overcoring hole around the central borehole to the starting position of overcoring
2. Drilling of the 200 mm borehole below the target overcoring (to break off core, if necessary) borehole to the starting position of overcoring
3. Drilling of small boreholes near the central borehole with a length of about 1.4 m from the start position with a 36 mm drilling rod
3/b. Installation of the long-sleeve single packer (3 m long) in the central borehole for sealing and to avoid resin flow towards the central borehole
3/c. Installation of resin packer system (incl. carbon fibre reinforcement) and performance of resin injection
4. Removal of the central borehole single packer and resin packer after resin solidification around the dummy
 Steps 3 to 6 were repeated twice
4/b. Drilling of the 200 mm borehole below the target overcoring (to break off core, if necessary) borehole to the end position of the overcoring
5. Drilling of 300 mm overcore from start position to end position
5/b. Breaking out and removal of the 300 mm core. (A single packer of 1 m sleeve length  was onsite available for retrieval of the core if it got stuck)

 
Long-Term Cement Studies (LCS) - Schematic display of overcoring procedure
Figure 1: Schematic display of overcoring procedure.
 
Drilling activities
 
Long-Term Cement Studies (LCS) - Drilling activities
 
Resin injection
 
The fractures were stabilised with resin (see below) to hold the rock pieces together during removal and transportation of the overcore sample. In order to trace the resin and see where it has penetrated during resin injection some fluorescent dye was added to the resin. 
 
Long-Term Cement Studies (LCS) - Resin injection
 
Core withdrawal and preparation on site
 
After withdrawing the core from the borehole, the large rock piece was geologically mapped and photographed. After that, the outer surface (incl. 86mm borehole) was sealed with resin to prevent evaporation from the core.
 
Long-Term Cement Studies (LCS) - Core withdrawal
 
Core cutting
 
The core was cut in smaller pieces to make it easier to transport and further analyse the rock in the laboratory.
 
Long-Term Cement Studies (LCS) - Core cutting
 
CT analysis of overcore
 
The different rock pieces were analysed by computer tomography (CT) to detect zones where grout can be found inside the rock. This information will be useful to determine areas which will be cut and analysed in more detail and with more sophisticated analytical techniques (e.g. SEM, EDX, Raman spectroscopy, element mapping).
 
Long-Term Cement Studies (LCS) - CT analysis of overcore
 
The result from the CT image analysis of a grouted fracture (after removing all sections with considerably different densities) shows distinct areas where grout has been observed as well from outside (see photograph on top right). Detecting grout fillings of finer fractures was not (yet) possible with this technique.

Long-Term Cement Studies (LCS) Experiment