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Open hole drilling

In stable formations, table tool holes are often completed without casing. Unstable materials are usually cased.

Table tool and bucket auger drilling can make use of mud fluids to stabilise unconsolidated formations. This technique is of particular advantage in large holes where eventual recovery of casing would be very difficult, but it can be used in any size hole.
Shell bits or ring bits which have a cutting action similar to that of an earth socket, are used in mud filled holes (Chapter 3).
Very heavy tool strings are necessary to get adequate falling velocity in mud. Large diameter drill stems must be made up with high torque to prevent joint failure caused by the bending forces arising from uneven drilling conditions over the large hole diameter.
Muds for non circulated holes
The prime function of these muds is to support the walls of the hole.
If the drilling is to lead to construction of a water well, the mud must:
* protect the aquifer,
* minimise time required for development.
Bentonite muds have been proved satisfactory for supporting the hale. Polymer muds will perform all three functions satisfactorily.
Bentonites are sometimes "thrown in" a table tool hole to improve mixing of cuttings. Bentonite, used in This way, provides cutting suspension and wall stabilisation much superior to that provided by most native clays.
Note: If this bentonite has not been hydrated before being added to the hole, it will block permeable formations, damaging them permanently.
Bentonite to be used for hole stabilisation, must be hydrated fully before being added to the hole. This is particularly so when the drilling could penetrate aquifers. Allow at least 12 hours for hydration, preferably 24 hours.
Tanks must be provided to allow cuttings to settle before the mud flows back to the hole. 

Source: http://www.miningandmetallurgy.com/mining/open-hole-drilling