Borehole Stability

General lack of cored shale material, as well as time consuming tests, requires utilization of small samples including even drill cuttings. We have developed several experimental set-ups to handle this. The picture shows the interior of cell used for triaxial measurements on small samples.

Based on the laboratory work, we have established an extensive shale property database which serves as input to simple correlations like the one shown above as well as more physically based models.

To better understand the physico-chemical interaction between shale and drilling fluid and how this can be used to improve the mud design, we perform specialized tests where shales are exposed to selected fluids.

The figure shows that for this Tertiary field shale, exposure to various brines at triaxial conditions leads to a shrinkage. However, the shrinkage not only depends upon the fluid activity, but also upon the type of ion present. This illustrates the importance of ionic exchange in shale-drilling fluid interaction.

Borehole failure is not equivalent to a hole stability problem. Whether the problem becomes significant in practice, and our ability to handle it, depends upon the failure mode.

The picture shows dedicated equipment used to run hollow cylinder tests, where drilled holes in shale plugs are brought to failure subject to various conditions. At the same time we perform numerical simulations that will be calibrated against these experiments.