BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
General
• Oil-based drilling fluids often allow a lower drilling fluid density to be used to prevent
collapse in shales. This provides a larger mud weight window.
• The risk of instability in highly laminated shales may be reduced when adopting a
trajectory normal to bedding.
Symptoms and Remedial Action
• The onset of cavings from a formation while it is being drilled may indicate underbalance
conditions. An increase in drilling fluid density or a reduction in ROP may help.
• The onset of cavings more than a few hours after drilling a shale indicates that the benefit
of the initial overbalance has been lost. This is a result of the migration of filtrate into the
formation causing near wellbore pressure increases. An increase in drilling fluid density
and/or a reduction in API filtrate are likely to help.
• Often an improvement in LOT value can be observed as the section is drilled. Consider
repeating the LOT where low values have originally been obtained.
• Even in normally stressed regions the mud weight window may be influenced by well
azimuth. Be prepared to increase the drilling fluid density in wells with azimuths subparallel
to the maximum horizontal stress direction.
• Controlling API filtrate to a minimum is particularly important in ERD wells and all hole
sections, not simply in the reservoir section.
• Swab and surge pressures may trigger instability in weak or highly fractured shales.
Particular care is required when running in and pulling out of hole sections with such
formations present. Pressure While Drilling (PWD) measurements indicate that surge
pressure equal to 1.5 ppg (0.18 S.G.) can be generated not just when tripping, but also on
connections made with a top drive.
• Unless absolutely necessary, do not reduce the drilling fluid density while drilling if a
shale is present in the open hole section, otherwise the risk of hole instability is greatly
increased. If operational difficulties necessitate a drilling fluid density reduction then the
slower this is done the better.
The Mechanisms of Wellbore Instability – Chemical Aspects
Chemical wellbore instability is due to chemical interaction between the formation being
drilled and the drilling fluid. This can occur in two main types of formation:
• Shales
• Salt formations
In both cases, it is an interaction with water which causes instability. Thus, chemical stability
is always minimized by using oil based drilling fluids.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-6