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JOUR.NALOF ENGINEERING AND APPLIED SCIENCE, VOL. 47, NO.5, OCT. 2000, PP. 965-979 FACULTY OF ENGINEERING, CAIRO UNIVERSITY THE EFFECT OF SHEAR FORCE ON THE STABILITY OF A SOLID FREE EMULSION MUD EMAD S. AL-HOMADHI ABSTRACT The degree of stability of a certain emulsion can be measured by several methods. An invert emulsion fluid is composed of brine emulsified into oil. The applied shear breaks the brine phase into droplets, which could be stabilized by the action of the emulsifier. The stability of the emulsion in most cases is linked to the homogeneity and fineness of the brine-dispersed droplets. In this study, solid-free emulsion muds were prepared from 25% CaCl brine emulsified into low toxic diesel oil. The effects of the shear rate and shear time factors on the stability of these muds were evaluated. Five muds with different oil/water ratios were tested under five different shearing rates and five different shearing times. Shearing rates of 1945 to 3359 sec-l were applied, and the shearing time was between 30 and 120 min. Oil/water ratio was varied from 90:10 to 50:50. Electric stability, filtration loss, separation test, and mud viscosity were used as the criteria that determine emulsion stability. The variation of filtration loss, electric stability, and mud viscosity versus shear force were quantified. Also the effect of oil/water ratio variation on these properties was tested. The results show a clear trend of the effect of shear rate and shear time on these properties and consequently on emulsion stability. Several correlations were developed to calculate the value of these properties and consequently the stability degree of the emulsion. KEYWORDS: Shear force, stability, emulsion mud, separation test and filtration loss. CONCLUSIONS 1. Several correlations were produced to simulate the effect of shear force and oil / water ratio on the emulsion stability factors. By using these correlations, these factors can be calculated to determine the stability degree or condition of the emulsion. 2. An excellent improvement in electric stability versus shear rate was observed. 3. Shear rate variation causes a similar response in the filtration loss for all tested emulsions. Increasing shear rate to the double decreased filtration loss to less than 20% which can be interpreted as a great improvement in the emulsion stability. 4. Shear time factor was less effective than the shear rate factor in producing more stable emulsion. 5. The calculated stability factors (using the produced correlations) versus the measured factors plot shows an excellent agreement between the two values. 6. At initial shear rate and time, phase separated rate decreases sharply seven times (in average) when the water ratio in the emulsion was increased from 20% to 40%, which is a superior improvement in emulsion stability. At the same initial shear time, doubling shear rate reduces phase separation further by more than four times. 7. Increasing brine ratio from 10 to 50% resulted in only 20% reduction in break down voltage.