BAKER HUGHES - Drilling Fluids Reference Manual

Oil / Synthetic Fluids
3. Lubricity
Torque and drag problems related to deviated well-bore, crooked hole, and sidetracks can be reduced
considerably with the use of oil-base fluids. The lubricating properties of an oil-base fluid are a result
of the chemical nature of the oil-wetting surfactants and the inherent lubricity of the oil itself.
4. Resistance to Chemical Contamination
Carbonate, evaporite, and salt formations do not adversely affect the properties of an oil-base fluid.
CO 2 and H 2 S can be easily removed with lime.
5. Gauge Hole in Evaporite Formations
When drilling salts, the internal (aqueous) phase of the oil-base system becomes saturated. However,
as the formation salts saturate the internal phase, it is not uncommon for the internal phase to become
super saturated with salt. This will result in the precipitation of excess salts. If calcium chloride is
precipitated, this will affect the stability reading negatively, but more importantly, it can cause waterwetting
of mud solids and de-stabilize the system. As a reminder, the electrical stability should only
be run on screened samples to remove any insoluble salts prior to testing; otherwise a false reading of
near zero will be recorded.
6. Solids Tolerance
Drill solids are inert in oil-base fluids and less dispersible other than from that resulting from
mechanical shear; therefore the fluids have a much higher solids tolerance. The key to solids tolerance
is less due to volume than it is to particle size and surface area of the solids. This is why oil-base
systems are more tolerant of solids resulting in reduced dilution requirements for solids control.
7. Reduced Production Damage
Oil-base fluids normally have a low fluid loss and the filtrate from a stable mud is generally all oil.
The filtrate penetration into the formation would be less than that of filter paper as the well-bore
surface area increases incrementally with distance from the well-bore. The filtrate from an oil-mud
invades only a short distance into the formation production zone and therefore minimizes damage.
8. Reduced Tendency for Differential Sticking
A thin, slick filter cake is formed when using an oil-base fluid, minimizing the chance of differential
sticking. Note, however, that differential sticking is still possible if drilling highly overbalanced,
especially with a high fluid loss system.
9. Drilling Under balanced
The wellbore stability resulting from the use of oil-base drilling fluids allows, in some instances,
drilling with under balanced pressure, thereby increasing the rate of penetration (ROP).
10. Re-Use
The excellent stability and solids tolerance of a well-conditioned oil-base fluid allows its use in
multiple wells. Although the initial make-up cost can be expensive, the re-use of oil-base fluids can
actually result in less expense than water-base fluids since the ability to recondition for multiple
applications exists.
11. Reduced Cement Cost
Gauge hole drilling can substantially reduce cement costs. This can be of benefit in two ways – (1)
less cement is required; and (2) a more constant flow regime can be achieved resulting in less fluids
contamination and a cleaner well-bore prior to cementing, thereby giving a better primary cement job.
Baker Hughes Drilling Fluids
Reference Manual
5-2 Revised 2006