10

150 250

TEMPERATURE °F a. When flocculated with 5 meq/liter of NaOH.

cl O

cl O

50 150 250 350

TEMPERATURE °F b. When flocculated with 50 meq/liter of NaOH. Note floe point is 15 meq/liter NaOH.

Figure 5-35. Effect of temperature on the plastic viscosity, np, and yield point (YP), on 4% monoionic sodium montmorillonite suspensions. (From Hitler.48 Copyright 1963 by SPE-AIME.)

viscosity of a bentonite suspension increased at both high and low shear rates after the suspension was rolled at high temperatures. The increase in viscosity at high shear rates must be ascribed to an increase in the degree of dispersion the greater increase at low shear rates was caused by increases in both the degree of flocculation and the degree of dispersion.

The behavior of suspensions of calcium clays at high temperatures is different from that of sodium clay suspensions, and considerably more complex. The interparticle repulsive forces of calcium clays are much weaker than those of sodium clays; consequently, the effect of high temperature on the degree of flocculation is much stronger, and thus even the plastic viscosity increases, as shown in Figure 5-39,

The effect of high temperature flocculation on viscosity and gel strength increases with increase in clay concentration. Figure 5-40 shows the increase in the 10-minute gel strength at 300°F (150 C) versus clay concentration for untreated bentonite suspensions and for suspensions treated with the optimum amount of lignosulfonate. 10-minute gel strengths at 300 F are also shown for a number of lignosulfonate-treated field muds versus their clay content, expressed as equivalents of bentonite, as measured by the methylene blue test (see Chapter 3). Note that the correlation is quite good. The slight scatter is probably due to the field muds not being fully deflocculated.

Figure 5-38. Effect of hot rolling on shear rate—shear stress relationships. (From Annis*7 Copyright 1967bySPE-AIME.)

SHEAR RATE—SEC 1

Figure 5-38. Effect of hot rolling on shear rate—shear stress relationships. (From Annis*7 Copyright 1967bySPE-AIME.)

High temperature behavior varies widely with the type of mud. For example, salt water muds are comparatively stable because the high electrolyte content prevents the clays from dispersing. The behavior of gyp-CLS muds is similar to that of the calcium montmorillonite suspension shown in Figure 5 -39. Lime muds, as already mentioned, develop high gel strengths becausc of the reaction between the hydroxide and the clay minerals, but calcium surfactant muds remain quite stable at temperatures up to 350 F (see Chapter 9).

The investigations of Hiller and Annis showed that accurate rheological parameters for water-base drilling muds at elevated temperature can only be obtained by direct measurement at the temperature of interest. However, the results shown in Figure 5-40 suggest that, for each mud type, correlations in the laboratory might be obtained that would enable approximate subsurface values for that type of mud to be predicted from wellsite tests at ambient temperatures.

50 150 250 350

TEMPERATURE °F

Figure 5-39. Effect of temperature and pressure on the yield point <f>, and plastic viscosity, of a 13% suspension of pure calcium montmorillonite to which 5 meq/liter of CaCI2 have been added. {From Hiller.46 Copyright 1963 by SPE-AIME.)

Figure 5-41. Effective viscosities of invert emulsion oil muds at several pressures. (From Combs and Whitmire.*6 Courtesy of Oil and Gas J.)

The effect of temperature and pressure on the rheology of oleophilic invert emulsion muds is almost entirely physical, and changes in the subsurface properties can largely be accounted for by the effect of temperature and pressure on the viscosity of the continuous phase, which is usually diesel oil. Combs and Whitmire46 measured the effective viscosities in a capillary viscometer at several temperatures and pressures, and found that, when the viscosities were normalized to the viscosity of diesel oil at the same temperature and pressure, the points al! fell on a single curve for each temperature (see Figure 5-42). They ascribed the small difference between the curves to a decrease in the degree of emuisilication at the higher temperature. These results show that the subsurface viscosities of this type of oil mud can be predicted from viscosities measured at ambient temperatures by means of a correction factor, based on the viscosity of diesel oil at the temperature and pressure of interest, provided that the muds remain substantially stable

0 0

Post a comment