Figure 9-1. Measurement of coefficient of friction, u = F/W.

where u is the coefficient of friction, F the force parallel to the surface, and the force normal to the surface. Note that u is constant for homogenous surfaces; thus, for a given value of Wu F is independent of the area of contact.

To evaluate lubricants for torque reduction, Mondshine1 used the apparatus shown in Figure 9-2. The steel test block simulating the wall of the hole is pressed against the test ring by a torque arm.* F is measured by the amperes required to turn the ring at a given rpm when immersed in the test mud. To get repeatable results, a steel test block was used. Mondshine found that although the coefficients of friction measured with steel blocks differed from those measured with sandstone or limestone blocks, the relative results obtained with different muds were substantially the same.

Results obtained with this machine were not in accord with some previously held notions. For example, it was thought that bentonite reduced torque because of its slippery nature. Test results showed that it did so only at low loads (less than 100 psi), and sharply increased the coefficient of friction at high loads. Similarly, it was believed that oil emulsified into a mud with an oil-wetting surfactant reduced torque. Test results showed that oil lightly stirred into the mud reduced friction considerably, but had no effect when tightly emulsified.

Table 9-1 shows the effect various water additives have on the coefficient of friction of water and two fresh water muds. These results were obtained under the standard conditions of 60 rpm and 150 inch-pounds (720 psi) load, which were judged to be representative of field conditions. The table shows that many of the agents reduced the coefficient of friction with water; some did so to a lesser extent with a simple bentonite mud, but only a fatty acid, a sulfurized fatty acid, and a blend of triglycerides and alcohols reduced friction in all the muds. These additives also reduced friction in a sea water mud.

The triglyceride mixture is one of the commercial water-soluble lubricity agents now commonly used in water-base muds to reduce torque. Oil muds are

* A lubricity tester in which the shaft can, if desired, bear against a filter cake is shown in Figure 3-24.

Figure 9-2. Lubricity testerfor drilling muds. (Courtesy of NL Baroid Petroleum Services.)

excellent torque reducers, presumably because of their oil-wetting properties; however, cost and potential pollution prevent the use of oil muds where the only advantage is torque reduction.

The fatty acid compounds referred to above are extreme pressure (EP) lubricants which were originally introduced by Rosenberg and Tailleur2 to decrease the wear of bit bearings. The action of EP lubricants differs from that of ordinary lubricants. Under extreme pressure, ordinary lubricants are squeezed out from between the bearing surfaces, and the resulting metal to metal contact causes galling and tearing. According to Browning,3 the lubricating properties of EP lubricants are due to the lubricants reacting chemically with the metal surfaces at the high temperatures generated by metal-to-metal contact. The reaction product forms a film which is strongly bonded to the metal surface and acts as a lubricant.

Recently, glass beads have been shown to reduce torque and drag.3a For instance, in a field test, 4 Ib/bbl of 44-88 micron beads reduced drag from 37,000 lb to 25,000 lb. The action is not clearly understood. The beads may act as ball bearings, or they may become imbedded in the filter cake and provide a low friction bearing surface.

Table 9-1

Comparison of Various Mud Lubricants*


Concen- Lubricity tration, coefficient lb/bblt Water Mud A+

Mud Bt


Diesel oil


Asphalt and

Diesel oil


Graphite and

Diesel oil

Suiturized fatty acid

Fatty acid

Long chained alcohol

Heavy metal soap

Heavy alkylate

Petroleum sulfonate

Mud detergent brand X

Mud detergent brand, Y

Mud detergent, brand Z


Commercial detergent

Chlorinated paraffin

Blend of modified triglycerides and alcohols

Sulfonated asphalt

Sulfonated asphalt and diesel oil

Walnut hulls (fine)

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