BAKER HUGHES - Drilling Fluids Reference Manual

HORIZONTAL AND EXTENDED REACH DRILLING
the polymers are also being added for shale stabilization and/or filtration control, the added
advantage of improved lubricity is even more significant.
There are numerous additives on the market today being sold as drilling fluid lubricants. Many of
these are of little value, and, in fact, can be harmful by adversely affecting other fluid properties
such as viscosity, filtration control, and indirectly, fluid cost. Thus, careful evaluation and pilot
testing should be carried out in selecting the best lubricant. Drilling fluid lubricants can be broadly
characterized as anti-wear (extreme pressure lubricants) and modified film lubricants.
Extreme pressure (EP) lubricants were developed to decrease bit bearing wear prior to the
development of sealed bearing bits. EP additives are primarily used today to decrease casing wear
at high-load points. They commonly contain modified fatty acids, which, according to Browning,
react with the metal oxide surfaces of the pipe through a chemi-sorption reaction to maintain their
lubricating action at high friction temperatures.
Bol reported that the commercial lubricants he tested partially prevented adhesive wear, but effects
were not reproducible in his full scale casing-wear tester. He correctly points out, however, that
wear and friction have to be evaluated separately.
Modified film lubricants, on the other hand, are considered general borehole lubricants, and
perform more effectively at lower loads. These form continuous films by absorbing layer upon
layer, with the thickness of the film making and breaking with the amount of load applied. They
may be completely displaced at a very high load point, such as might occur between drillpipe and
casing in an high-angle portion of the well, but in general, they show sufficient tenacity to serve as
a lubricant between the drillstring and the open hole. Concentration effects are very important for
most of these compounds, and normally at least 2% by volume are required before any significant
improvement is observed. Most of these additives are hydrocarbon-based, though recent
developments have made available formulations to meet non-hydrocarbon environmental
regulations.
Filter Cake
Although much of the exposed wellbore will usually consist of shales with very low permeability, a
large portion will be shale-sands and sandstones with fluid filter cake deposited on their surfaces.
Thus, the lubricity coefficient between the pipe and filter cake becomes important.
Most drilling personnel characterize a desirable cake as one that is thin and tough with a leathery
texture. This describes a cake that is highly deflocculated, with relatively low water content. In
effect, this leathery surface results in a high-lubricity coefficient with the surface of the cake
resisting shear.
Certainly a thin cake is desirable, but it is even more important to have a cake where its outer
surface will shear or glide along with the rotating pipe surface. To obtain this property, the filter
cake must have relatively high water content, be highly compressible, and possess a very slick
surface.
In permeable formations it is essential that the drilling fluid contain the correct concentration of
bridging materials and that the bridging material (often calcium carbonate) has a particle size
distribution suitable for the formation being drilled. This will help to minimize the thickness of the
filter cake deposited and in so doing will aid in minimizing friction.
Wyoming bentonite tends to give this characteristic in fresh water under light loads, but in the
presence of drilling contaminants such as gypsum and salt, or in a highly deflocculated
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006. 11-20