Page 263 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 239

Drilling Fluids Program [ ]
2.5.8

defined as the ratio of the volume percentage of oil in the liquid phase
to the volume percentage of water in the liquid phase.
Many of the components of an invert emulsion oil mud are present
or perform in the oil phase. Consequently, if the oil phase is reduced
because the oil/water ratio has been reduced (in favor of water) or
because the solids phase has increased due to increase in density or
solids, the performance of some of these components will be affected.
For example, the yield point and gels of the mud can be raised with
the addition of viscosifiers. These viscosifiers are present in a whole
mud concentration but only act in the oil phase. If the oil/water ratio
is increased, the viscosity will decrease (unless additional viscosifier is
added) because the viscosifier concentration in the oil phase will have
been reduced. The viscosity will also have been reduced because the
water phase (and its contribution to viscosity) will have been reduced.
This effect is an attribute of an invert oil emulsion mud. It allows
for a stable control of viscosity by altering the oil/water ratio to counter
the tendency for viscosity to increase as fluid density is increased.
Normally a low-density oil mud will have a lower oil/water ratio than
one with a higher mud weight.
Determination of oil/water ratio. Oil water ratios can been engi-
neered to as low as 40/60 or as high as “all oil” (100/0) and still pro-
vide a stable mud. Very low oil/water ratios have sometimes been used
to provide a mud that will give a low concentration of oil on cuttings.
All oil systems have been used as drill-in fluids to study formation
water without any contaminating effect of water. They are sometimes
described as “native state coring” fluids.
Normally when using oil mud on a complete well, the shallower
intervals with lower mud weights would be drilled with oil muds hav-
ing 60/40 to 70/30 oil/water ratios. Deeper intervals with higher fluid
densities would see oil water ratios in the range of 75/25 to 85/15.
Specific consideration of the oil/water ratio should be made in
respect to any contaminating fluids entering the mud due to an influx.
If this is liable to happen, it is better not to have too small a water
phase. The contaminating influx will show more effect.
OWR also depends on density. The higher the density, the higher
the OWR required. Table 2-2 provides the lowest OWR recommended
for each density range.
The mud should be built with an OWR corresponding to the high-
est density that might be needed during drilling; otherwise, a density

239