BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
When shales react with water, they can soften, disperse, swell and crack. These effects can
cause a wide range of operational problems such as tight hole, hole enlargement, ledging, bit
balling and caving.
To minimize these problems, it is important to characterize the shale type at the planning stage
of a well, and to use an appropriately designed drilling fluid.
In salt formations, chemical instability occurs if the formation is soluble in water. The use of an
incorrectly formulated fluid will lead to uncontrollable washouts in these situations. Formation
types which exhibit this behavior are:
• Halite (NaCl)
• Carnallite (KMgCl 3 .6H 2 O)
• Bischofite (MgCl 2 . 6H 2 O)
• Sylvite (KCl)
• Polyhalite (K 2 Ca 2 Mg(SO 4 ) 4 . 2H 2 O)
Preventative Action – Chemical
Pre Drilling
• When planning a well, first decide if shales or salts will be encountered. Offset well data
and drilling fluid reports will be particularly useful.
• Design the casing/well program to minimize the length of time reactive formations are
exposed to the drilling fluid. Because shales have very low permeability (10 -9 – 10 -6 D),
they may appear stable for a time, but water can penetrate leading to time delayed effects.
This process is known as pore pressure transmission and can be countered by the use a
cloud point glycols, silicate and aluminate based drilling fluids.
• Characterize shale types by XRD (X-ray Diffraction) analysis. This technique should also
be supported by laboratory inhibition tests, which are best done on preserved shale.
• Watch out for inter-bedded formations (e.g. salt stringers in shale or reactive shale in
competent shale). A drilling fluid system compatible with both formation types will be
required.
• The best way to minimize chemical instability in shales or salt sections is to use an oil
based drilling fluid. High performance water based drilling fluids should be given serious
consideration as alternatives. A cost/benefit analysis should aid the final selection
process.
• Do not rely on chemical - mechanical wellbore stability models to design the drilling
fluid. There is invariably insufficient input data, and they do not take account of specific
chemical reactions.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-7