BAKER HUGHES - Drilling Fluids Reference Manual

HORIZONTAL AND EXTENDED REACH DRILLING
Figure 11 - 9
Comparison of Lubricity Coefficients – Baroid Lubricity Tester
Note that the lubricity coefficient of the oil-base fluid is relatively low. This fact correlates well
with field operations where oil fluids generally show good lubricating properties.
However, as noted in the section on borehole stabilization, oil-base fluids may not prove a viable
option where formations are prone to loss of circulation, or environmental regulations prohibit their
use. It is interesting to note that oil tightly emulsified in a water-base fluid (oil in water emulsion)
generally imparts very little lubricity to the system. Water is obviously the external wetting phase
and the emulsified oil droplets are not available for lubricating purposes. Diesel or mineral oils
contain no chemically-reactive components for boundary lubrication.
It is generally believed that Wyoming bentonite forms the basis of a good lubricating fluid because
of its slippery nature. At light loads this appears to be the case, but at loads comparable to field
conditions, the film strength of the clay is inadequate and very high coefficient of frictions can be
observed. As noted in Figure 11-9, the clay-based lignosulphonate fluid shows a relatively high
coefficient of friction. Although this type of fluid has been the most prevalent water-base fluid used
for the past 25 years, from a lubricity standpoint, it is not the best choice for an extended-reach or
horizontal well.
As noted in Figure 11-9, the polymer-based fluid, which contains 3 lb m /bbl of a high molecularweight
partially-hydrolyzed polyacrylamide, shows a significantly lower coefficient of friction than
that of the clay-based lignosulphonate fluid. This improved lubricity of the polymer-based fluid is
inherent in the polymer's behavior. The long chain polymers slide by one another creating a
boundary layer between the two sliding surfaces. Modifications of the vinyl co-polymer have also
been used to gain additional lubricity. In fact, combinations of these type of polymers with special
lubricants are beginning to show friction coefficients approaching those of oil fluids (see Figure
11-9). Fortunately these fluids can be formulated to meet strict environmental regulations. Since
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006. 11-19