Equipment And Procedures For Evaluating Drilling Fluid Performance

Devising tests that will accurately describe how drilling fluids behave downhole is virtually an impossible task. Most drilling fluids are complex mixtures of interacting components, and their properties change markedly with changes in temperature, shear rate, and shear history. As they circulate through the borehole, drilling fluids are subjected to ever changing conditions—turbulent flow in the drill pipe, intense shearing at the bit, and nominal laminar flow in the annulus at frequently changing shear rates (owing to changes in the gauge of the borehole). The viscous properties of most muds are time-dependent, and they seldom have time to adjust to any one set of conditions. In addition, there is a continuous temperature change as the mud circulates, and a continuous composition change as solids and liquids from the formation are incorporated into the mud by the drilling

Another problem is that tests at the wellsite must be performed quickly and with simple apparatus. To a lesser extent, this limitation also applies to laboratory tests made in support of field operations.

It is not surprising, therefore, that the standard field and laboratory tests which have been accepted by industry are quick and practical, but only approximately reflect downhole behavior. Nevertheless, these tests serve their purpose very well if their limitations are understood and if the data obtained from them are correlated with experience.

In this chapter, these tests are briefly described, and their purpose and underlying principles are explained. Detailed laboratory procedures are not given. These will be found in the API publication RP 13B,1 and it would serve no purpose to repeat them here.

Over the years, various tests that more closely simulate down hole conditions have been devised by individual investigators. These tests require-more elaborate equipment, are time consuming, and are therefore more ■suited to research and development. In many cases, the results from these tests are discussed in subsequent chapters; in this chapter, the apparatus and procedures will be briefly described and the appropriate references given.

Sample Preparation

Since the properties of muds depend so much on shear history and on temperature, it is of the utmost importance that muds to be tested in the laboratory first be subjected to conditions similar to those prevailing in the drilling well. Muds made from dry materials should be given a preliminary mixing, and should then be aged for a day or so to allow the colloids time 10 hydrate. Then, the mud should be subjected to a high rate of shear until a constant viscosity is obtained, and their properties tested at ambient temperature. If the mud is intended for use in a well with a bottom-hole temperature greater than 212°F (100°C), it should be aged at the temperature of interest, as outlined later in this chapter.

Samples brought in from a well will have had time to cool, and, if thixotropic, will have set up a gel structure. Such muds must be sheared at the temperature observed in the flow line until the viscosity corresponds to that measured at the rig.

Mixers such as the Hamilton Beach (Figure 3-1) and the Multimixer (Figure 3-2) are used in laboratory tests of mud materials. They do not, however, produce the high rates of shear that exist in circulation of the drilling fluid in wells. High rates of shear are only obtained when there is little clearance between the stator and the rotor, or when the mud is pumped through a small orifice or opening. Food blenders in which the blades rotate in a recessed section at the bottom of container (Figure 3-3) provide high shearing rates, and are suitable for shearing small quantities (about a liter) of mud. They can only be used for short periods of time because the temperature rises rapidly, with conseqent loss of water by evaporation. For larger amounts of mud, it is best to use a high-shear mixer such as the Eppenbach. This instrument consists essentially of a circulating unit which is mounted on rods extending from the base of the driving motor. This arrangement enables the unit to be lowered into a large vessel (8 liters or so) of mud, and to circulate the mud throughout the vessel. The clearance between the rotor blades and the baffles on the housing is close so that a high rate of shear is maintained in the circulating unit.

Figure 3-1. Hamilton Beach mixer and cup. (Courtesy of NL Baroid.)

Alternatively, a colloid mill may be used to achieve an extremely fine state of subdivision. Sinha and Kennedy2 obtained excellent reproducibility with an Eppenbach colloid mill, model Q-V-6-3, modified so that the mud was circulated through a cooling coil and evaporation was prevented.

Another excellent way of pre-shearing muds is to pump them around a closed circuit containing a shearing valve or other restriction, and a hea! exchanger. A description of such systems is given in the section on multifunctional systems later in this chapter.

Figure 3-2. Multimixer. Uses same cups as Hamilton Beach. (Courtesy of NL Baroid.)
Figure 3-3. High speed blenders. Narrow clearance between blades and baffles results in high shear rates. {Courtesy Fann Dresser.)
Figure 3-4, Balance arm for determining mud density. Graduated in lb. per cubic foot, lb. per gallon, lb. per square inch, per 1000 feet, and specific gravity. (Courtesy of NL Baroid.)

Viscosity

The Marsh Funnel. This instrument is useful on the drilling rig. where it enables the crew to periodically report the consistency of the mud, so that significant changes may be noted by the mud engineer, it consists of a funnel and a measuring cup (see Figure 3 -5), and gives an empirical value for the consistency of the mud. The procedure is to fili the funnel to the level of the screen and to then observe the time (in seconds) of efflux of one quart (946cc). The number obtained depends partly on the effective viscosity ai the rate of shear prevailing in the orifice, and partly on the rate of gelation. The time of efflux of fresh water at 70 + 5 :,F (21 ±3'C) is 26 + 0.5 seconds.

Figure 3-5. Marsh funnel and measuring cups. (Courtesy of NL Baroid.)

Figure 3-6. Schematic diagram of the direct indicating viscometer. The deflection in degrees of the bob is read from the graduated scale on the dial. (Courtesy of J.D. Fann.)

Direct-Indicating Viscometers. These instruments are a form of concentric cylinder viscometer that enable the variation of shearing stress with shear rate to be observed. The essential elements are shown in Figure 3 -6. A bob suspended from a spring hangs concentrically in an outer cylinder. The assembly is lowered to a prescribed mark in a cup of mud, and the outer cylinder rotated at a constant speed. The viscous drag of the mud turns the bob until balanced by the torque in the spring. The deflection of the bob is read from a calibrated dial on the top of the instrument, which thus provides a measure of the shear stress at the surface of the bob.

Dresser.)

The direct-indicating viscometer is available in several forms. The Fann V-G meler, model 34 (Figure 3-7), has two constant speeds: 600 and 300 rpm. The Baroid rheometer is similar, but is hand cranked instead of motor-driven. The two-speed viscometer was designed so as to enable the Theological parameters to be easily calculated.4 The plastic viscosity (PV). in centipoises, is calculated by subtracting the 300 rpm dial reading from the 600 rpm reading, and the yield point (VP), in lb/100 ft- (multiply by 0.05 to obtain kg m2), is obtained by subtracting the PV from the 300 rpm reading

Dresser.)

direct indicating viscometer. (Courtesy Fann

The apparent viscosity (AV), in centipoises, is obtained by dividing the 600

rpm reading by 2. The power law constants are calculated as follows;

600 rpm reading 300 rpm reading

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