BAKER HUGHES - Drilling Fluids Reference Manual

Oil / Synthetic Fluids
Solids control with oil mud
The removal of solids in oil muds is different than in a water based mud. Cuttings generated by the bit
tend to remain more competent, making the shale shaker a much more efficient solids removal device.
At the same time, due to the viscosity of the oil phase, hydrocyclones and centrifuges become less
efficient. The waste generated by these devices also contains a high percentage of oil mud which can
result in disposal problems.
The problem becomes one of preventing solids build up in the system. It is therefore imperative to
maximize the use of solids control equipment to remove as much of the unwanted solids as possible.
Drill solids that are not removed by solids control equipment must be diluted. Dilution not only is
very expensive, but it creates mud volume control problems.
Oil muds have a very expensive liquid phase. This phase consists of base oil, brine, and surfactants.
As we increase the density of oil muds, the cost of the solids phase goes up, but the liquid phase
continues to be expensive. This forces us to re-think our usage of equipment. Refer to for an
example of oil mud costs vs. density.
In water base muds, the liquid phase is expensive when unweighted. As we weight up, at some
density the cost of the barite becomes more expensive that the liquid phase. At higher density, we
now concentrate our solids control on barite recovery. This is never the case with oil muds.
Solids
The removal of drill solids is important for the control of any mud system. It is of particular
importance in water based fluids because of the magnitude and variety of adverse effects the solids can
have on the system. However, in oil based mud systems, solids have historically been only
superficially considered. This is primarily the result of the fact that in properly conditioned fluids
there are no active drilled solids.
It has become apparent through the use of relaxed filtrate oil based fluids that when rate of penetration
is considered, solids have the same detrimental effect in oil based fluids as they do in water based
muds. As the solids content increases, the rate of penetration decreases. In addition, if the solids
remain in the system, they require treatment with oil wetting agents to maintain fluid properties and
stability, thus adding to the expense of the system.
If solids do build up, the standard technique of dumping and diluting is not viable because of the
expense involved in dilution and the expense involved with disposal of the waste mud. Treatment of
the system to reduce the effects of solids on flow properties is effective to a certain point. Surfactants
can be added and the oil-water ratio can be increased to partially offset the effects incorporated solids.
Nevertheless, a point will be reached where the colloidal solids content will no longer be affected by
surfactants and the oil-water ratio can no longer be effectively increased. When this occurs, the rate of
penetration of the fluid will already have been reduced to that of a conventional mud and the control of
rheological properties can only be accomplished by dumping mud and diluting with oil.
Baker Hughes Drilling Fluids
Reference Manual
5-46 Revised 2006