10

Figure 5-32. Determination of Z parameter in annular flow. (From Walker,37 Courtesy of Oil and Gas J.)

fied conditions in the well by means of Equation 5-48. Such tests have shown that friction reducers can reduce pressure losses by as much as a factor of three.39 Unfortunately, the beneficial effect of the polymer is lost if the fluid becomes contaminated with substantial amounts of clay.

Based on data derived from tests made with full-scale laboratory equipment, Randall and Anderson398 developed equations relating/to NRe and to Fann viscometer 600 and 300 readings. These equations were found to predict the pressure losses of polymer fluids—as well as other muds—in the field with acceptable accuracy.

Friction reduction must be distinguished from shear thinning; they are I wo quite different phenomena. Shear thinning, as we have seen, results from a reduction in structural viscosity. The mechanism of friction reduction is not known for certain, but it appears to result from the elastic properties of the long-chain polymers, which enable them to store the kinetic energy of turbulent flow.38

Figure 5-33. Effect of carboxymethyl cellulose on the Fanning friction factor. (From Dodge and MetznerCourtesy of A.I.Ch.E.)

REYNOLDS NUMBER, N'Re

Figure 5-33. Effect of carboxymethyl cellulose on the Fanning friction factor. (From Dodge and MetznerCourtesy of A.I.Ch.E.)

Influence of Temperature and Pressure on the Rheology of Drilling Fluids

The rheological properties of drilling muds under downhole conditions may be very different from those measured at ambient pressures and temperatures at the surface. At depth, the pressure exerted by the mud column may be as much as 20,000 pounds per square inch (1,400 kg/cm2). The temperature depends on the geothermal gradient, and may be more than 500°F, (260°C) at the bottom of the hole during a round trip. Figure 5-34 shows estimated mud temperatures during a normal drilling cycle in a 20,000 foot (6,100 meters) well.40 Even quite moderate temperatures can have a significant, but largely unpredictable influence on the rheological properties. Muds may be thicker or thinner downhole than indicated at the surface, and an additive that reduces viscosity at the surface may actually increase the viscosity downhole.41

Elevated temperatures and pressures can influence the rheological properties of drilling fluids in any of the following ways:

1. Physically: An increase in temperature decreases the viscosity of the liquid phase; an increase in pressure increases the density of the liquid phase, and therefore increases the viscosity.

2. Chemically: All hydroxides react with clay minerals at temperatures above about 200°F (94 O. With low alkalinity muds, such as those treated with caustic tannate or lignosulfonate, the effect on their rheological properties is not significant, except to the extent that the loss of alkalinity lessens the effectiveness of the thinner. But with highly alkaline muds the effect may be severe, depending on the temperature and the species of metal ion of the hydroxide.42 In the notorious case of high-solid lime-treated muds, hydrated alumino-silicates were formed, and the mud set to the consistency of cement at temperatures above about 300 F (150 C).4i 3. Electrochemically: An increase in temperature increases the ionic activity of any electrolyte, and the solubility of any partially soluble salts that may be present in the mud. The consequent changes in the ionic and base-exchange equilibria alter the balance between the interparticle attractive and repulsive forces, and hence the degree of dispersion and the degree of flocculation. (See Chapter 4). The magnitude and direction of these changes, and their effect on the rheology of mud, varies with the electrochemistry of the particular mud.

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