BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
• Many believe that the use of an invert emulsion drilling fluid will prevent any problems
occurring while drilling in shales. Hole instability can still occur particularly if the
drilling fluid density or water phase salinity is inappropriate.
• It must be recognized that drilling fluid recommendations based on theoretical models are
liable to be unreliable and often require “fine tuning” to give sensible, practical drilling
fluid densities. Area experience is a better guide.
• Too great of an emphasis is often placed on annular velocity in the process of hole
enlargement. Reducing API filtrate and increasing inhibition and overbalance will
often have more beneficial effects than reducing annular velocity.
The Mechanisms of Wellbore Instability – Mechanical Aspects
Unconsolidated Formations
This type of formation would usually be associated with top hole intervals, but may also be
encountered in fault zones or in unconsolidated reservoirs. Unconsolidated formations have no
cohesive strength. Consequently when they are drilled with a clear fluid that exerts no
confining stress on the wall of the hole the formation will slough into the hole. Most commonly
the unconsolidated formation will be sand. However, in some tectonically active areas, a fault
zone will be encountered that contains rock flour and unconsolidated rubble.
Preventative and Remedial Action
• Drilling this type of formation with a drilling fluid that has good filtration characteristics
will produce a filter cake on the rock. The pressure drop across this cake will impart
cohesive strength and a gauge, or near gauge, hole can often be achieved. The drilling
fluid should contain bridging solids (usually calcium carbonate or fibrous seepage loss
material) to promote the rapid build-up of a filter cake. If a cake is not quickly
established the turbulent flow at the bit will produce washed out hole.
• Use the minimum flow rate that will clean the hole to prevent the erosion of the filter
cake.
• Consider the use of a drilling fluid with good low shear rate viscosity so that high pump
rates are not required. Xanthan gum polymer (XAN-PLEX ® D) and mixed metal
hydroxide (MMH) have an application in these cases.
• Do all that is possible to avoid the mechanical removal of the filter cake – minimize trips,
minimize reaming and back-reaming, avoid rotating stabilizers next to the unconsolidated
formation.
• The use of a drilling fluid known to have enhanced fracture sealing capabilities may help
to stabilize fault zone rubble beds. Specific drilling fluid systems and additives can be
designed for this application.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-2