Combination EP and Lubricity Tester Model EP-C
Frictional resistance to rotation of the drill string is called torque, and frictional resistance to hoisting and lowering the drill string is called drag. Many different materials have been used as drilling fluid additives to improve lubricity, thereby reducing friction. The lubricity, or drill string to bore hole wall drag, of drilling fluid is a property of special importance in drilling directional wells. An increase in friction between drill string and borehole is expected when drilling a hole off-vertical. Most of the wells drilled from fixed platforms offshore are completed in deviated holes. Desirable characteristics of a lubricant for this purpose, aside from the obvious requirement of performing well as a lubricant, are that it is non-toxic and bio-degradable, and does not form an oily slick on water. Since evaluation of the various lubricating materials in the various types and quality of drilling fluid cannot realistically be done on the drilling rig, a functional (drilling fluid lubricity) test was designed to simulate the torque and drag produced by a given drilling fluid downhole. The tester models or approximates the speed of rotation of the drill pipe and the pressure with which the pipe bears against the wall of the hole where the friction is generated.
Extreme pressure lubricants have been developed to increase the life of bit-bearings. Extreme pressure lubrication deals with metal surfaces in rubbing contact with each other at very high pressures (30,000 to 100,000 psi (206,820 to 689,400 kPa)). Lubrication for the metal surfaces is provided by a pressure resistant film that is produced as a result of the chemical reaction initiated by a high temperature generated from friction at the area of contact.
The combination EP (Extreme Pressure) and Lubricity Tester is a high-quality instrument designed to measure the lubricating quality of drilling fluids, provide data to evaluate the type and quantity of lubricating additives that may be required, and predict wear rates of mechanical parts in known fluid systems.
The EP tests (Fig. 1) are performed by applying a measured torque with a torque arm to a torque-sensitive, rotating bearing cup. This provides a means of testing lubrication under extreme pressure conditions and produces an indication of the film strength of the fluid being tested. The problem of reduction of friction between the drill string and the borehole requires a different simulation. The more common lubricity test (Fig. 1) measures fluid resistance (lubricating character) between two hardened steel moving surfaces at 100 pounds force (which translates into a 5,000 to 10,000 psi (34,470 to 68,940 kPa) pressure on the intermediate fluid film). During the lubricity test, a steel block is pressed against a rotating steel ring. Load in inch-pounds (in-lb) is read directly from the dial on the torque arm.
Measure of friction is a requirement for the determination of the film strength of a lubricant, for bit bearing wear, as is obtained in EP test and for the determination of torque or drag of the drill pipe as determined in the lubricity test.
Friction is measured as the coefficient of friction (µ). The coefficient of friction (µ) between two solids is defined as F/W, where F denotes the frictional force and W is the load or force perpendicular to the surfaces.
Thus, the coefficient of friction is independent of the apparent area of contact as long as this area is not so small as to break through the film; that is, with the same load W, the force to overcome friction will be the same for a small area as for a larger area.
Applied to the Lubricity Tester, the load is the force with which the test block is pressed against the test ring through the torque arm. The force F required to slide the block and ring surfaces across each other at a given rate is measured by the power required to turn the test ring at a prescribed number of revolutions per minute. The coefficient of friction, µ = F/W. Refer to Figure 1.
Figure 1 – Comparison of EP and Lubricity Test