BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
Oil-Based Drilling Fluid –Engineering Comments
• Oil-based mud (OBM) water phase salinity must be at least as high as the pore fluid
salinity of the shale. This will prevent water entering the shale by osmosis.
• When drilling salt formations, OBM salinity should be high (e.g. 300,000 mg/l chloride),
to minimize salt dissolution into the water phase of the drilling fluid.
• Synthetic-based mud (SBM) (pseudo oil based drilling fluids) should be considered
where environmental constraints restrict the use of conventional oil. Shale inhibition is
equally effective in these systems.
• In micro-fractured shales, use a very low HPHT filtrate drilling fluid (< 3 ml), and add
fracture sealing additives.
Water-Based Drilling Fluid –Engineering Comments
• If water-based mud (WBM) is to be used, carry out a screening program at an early stage
to allow optimization.
• Water based drilling fluids are less lubricating generally than oil based drilling fluids,
therefore expect higher torque in high angle wells. It may be necessary to add lubricants
to the system.
• In salt sections, it is important to match the fluid to the type of salt. Salt saturated drilling
fluids (NaCl) are used for simple halites; mixed salt systems are available for complex
salts such as Carnallite.
• Use a low filtrate drilling fluid (e.g. API < 5ml, HPHT (250°F < 14 ml) in microfractured
shales, and add fracture sealing additives.
Symptoms and Remedial Action
Having planned the well using all available data, the risk of mechanical and/or chemical
borehole instability will be limited. It is important that should instability occur it should be
identified and suitable remedial action should be adopted.
Indication of the condition of the hole can be inferred from torque and drag measurements, the
condition and quantity of cuttings seen at the shale shakers and variations in drilling fluid
volumes.
• When drilling shales, monitor cuttings quality as a qualitative measure of inhibition. Very
soft cuttings will mean insufficient chemical inhibition or, in the case of OBM, would
suggest that the water phase salinity is too low.
• High torque values would suggest tight hole, possibly requiring an increase in drilling
fluid density or an increase in inhibition to reduce the swelling of clays.
• A sudden appearance of large or increased volume of “cuttings” at the shale shakers is
indicative of wellbore caving.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-8