3. Introduction
Shales can be defined as clay-rich sedimentary rocks. They are sensitive to both mechanical and chemical
influences and so are particularly problematic to drill.
A useful classification of shales can be made in terms of hardness. Soft shale's (claystone/mudstone ) tend to
be relatively young and are usually located near to surface. Harder shales are older and usually more deeply
buried. Each type can be associated with a particular set of drilling problems, as summarised in the table
below. For example, soft shales are more likely to cause tight hole, bit balling and hole erosion problems.
Hard or brittle shales suffer more from sudden cavings and breakout due to failure at the wellbore wall. It is
also seen from the shale that hardness correlates with other properties such as MBT, water content and
shale density.
In tackling drilling problems in shales, both mechanical and chemical aspects should always be borne in mind.
The following provides a brief summary of the key points:-
5. Mechanical Effects:
There are a large number of
variables. Those which can be
controlled, and have a
significant influence over hole
stability are:
Of these, mud weight is
probably the most fundamental
with respect to hole condition.
If the mud weight is too low,
tight hole or wellbore breakout
can occur. Too high a mud
weight can fracture the
formation.
6. Chemical effects
Water will hydrate the clay minerals in shale. This can lead to
swelling, softening and dispersion in softer types, or rock failure
(fracturing) in the harder materials. If the shale is physically
constrained so that it cannot swell, significant swelling pressures can
develop.
It is crucial to minimise the reaction with water (i.e. create good
inhibition) utilising appropriate drilling fluids. For water based muds,
this is done using a combination of mud additives such as salts and
polymers. Salts, e.g. kcl, reduce clay swelling and dispersion, and
polymers may encapsulate the shale to hold it together (e.g. PHPA*)
or have a chemical strengthening effect (e.g. glycols). The action with
water can be eliminated completely by using oil based mud, but the
salinity of the mud must be sufficient to prevent water entering the
shale by osmosis.
