BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
Fluid Density
Insufficient drilling fluid density can also be a contributing factor. Care should be taken,
however, in how much the drilling fluid density is increased since differential sticking and loss of
circulation can result from excessive hydrostatic pressure.
LUBRICITY
Control of drilling fluid lubricity becomes extremely important in holes experiencing severe
torque and drag. These problems are most pronounced in a directional or excessively deviated
hole, and considerable effort has been directed to their solution. The application of extremepressure
(EP) lubricants in drilling fluids was first conceived in 1958. These lubricants were
originally introduced to increase bit life but, as work progressed, noticeable reductions of torque
and drag expanded the development of lubricants for aqueous fluids.
The problem associated with developing effective lubricants derives from the fact that some
lubricating products form a good boundary layer of protection but, under extreme pressures and
the associated high temperatures produced from friction, these lubricants breakdown. A
comprehensive testing of lubricants should include the use of an EP tester in combination with a
sticking coefficient apparatus. The EP tester cannot simulate conditions that are found when the
drillpipe contacts filter cake, but an approximation can be made using a stickometer which uses
actual filter cake to conduct the test.
Laboratory experience indicates that the Falex Lubricant Tester is a reliable laboratory test to
predict effectiveness of lubricants under extreme pressure conditions (see Fluids Testing
Procedures Manual). With the Falex Tester, properties are evaluated by comparing torque
measurements at varying increments of loading among a series of test samples, including a base
sample. Reductions in torque as compared to the base fluid are generally indicative of lubricant
effectiveness. The Falex Lubricant Tester has been modified for use with drilling fluids. This
device measures the torque generated between two steel surfaces in the presence of drilling fluid.
The readings are converted into coefficient of friction.
Laboratory measurements are valid in selecting lubricants, but these results alone will not always
predict effectiveness under downhole conditions. Variables such as temperature, quantity and
nature of solids generated and long-term effect of the drilling fluid in use are often difficult to
analyze. The ultimate test is obviously the results achieved under actual drilling conditions.
Some general conclusions obtained from laboratory data and substantiated by field experience are
noted below.
• Minimum concentrations of drilled solids generally result in improved lubricity.
• The effectiveness of diesel oil as an EP lubricant is greatly reduced when it is tightly
emulsified in a system. Diesel oil offers maximum lubricity when added continuously in small
quantities to the suction pit while drilling. The information on diesel oil obtained from
laboratory testing using the lubricity tester differs from results obtained using the sticking
coefficient instrument. This device indicates that emulsified oil in a water-base fluid system
does impart lubricity. This is most likely due to the oil becoming embedded in the filter cake
and thus lubricating the filter cake. Diesel oil emulsified with a surfactant to preferentially oilwet
metal surfaces will thereby impart fair lubricity. Surface-active materials are generally
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-43