BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
• Allow for increases in drilling fluid density of between 0.5 ppg and 1.0 ppg per 30
degrees inclination through shale/mudstone sections to combat hole collapse. Only local
experience will determine at which end of the scale to be.
• No increase in drilling fluid density with hole inclination is necessary across permeable
formations, e.g. sands. Formations with reasonable matrix permeability can usually be
drilled with nominal overbalance, regardless of well trajectory or formation strength.
• Be aware that the fracture gradient may reduce with increased angle.
Fracture Gradient
• Recognize that the fracture gradient for a hole section is more likely to be controlled by a
carbonate or sand rather than the shale within which the LOT was performed (see Figure
7 - 2).
• On ERD wells, drilling high pressure reservoirs may prove extremely difficult due to a
very tight mud weight window between taking a kick and experiencing losses. The extent
and effect of ECDs need careful consideration at the planning stage.
• During appraisal, consider performing micro-fracture tests (essentially a LOT taken
beyond the point of breakdown) to better determine the fracture gradient in formations
that may prove to be critical in an ERD well.
Regional Stress State
• Process any dipmeter or borehole imaging log data to determine in situ stress directions.
This may help to interpret any problems seen during the drilling operation and thus
hasten corrective actions.
• In highly tectonically stressed regions, drilling up dip of the major faults may provide a
larger mud weight window than drilling down dip, cross dip or vertically.
• The in situ stress state near a salt diapir is highly disturbed, such that well trajectories
which approach the diapir normal to its surface provide a larger mud weight window than
trajectories tangential to its surface.
Casing Program
• Having planned for an increased drilling fluid density to control shales in an ERD well,
assess whether the planned casing setting depths still provide sufficient mud weight
window.
• In ERD wells the drilling fluid density required to drill a normally pressured reservoir is
often significantly less than that required to prevent collapse in the cap rock. The setting
of the production casing should minimize or exclude the presence of cap rock in the
reservoir hole section, thus allowing the reservoir to be drilled with a normal
overbalance.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-5