In this study the effect of interactions among the ingredients of synthetic-based fluids (SBFs) on the rheology of the fluid at ambient temperature was investigated. It has been also attempted that through examination on the important variables affecting those interactions, the mechanism of viscosity building in SBFs be identified. Rheology measurements along with the microscopic observations from the invert emulsion structure of the SBFs can be utilized for such an examination. Viscosity building in the SBFs by means of organophilic clays is a function of several parameters including the type and amount of the clay, the synthetic based fluid, the composition of the internal dispersed phase, mixing time, and the type of emulsifier.

The impact of the synthetic-based fluid, i.e. an ester, olefin, or a paraffin, organophilic clays, water, calcium chloride, emulsifiers, and the mixing time on the rheological behavior of the produced mud was thoroughly investigated. Micrographs from scanning electron microscope (SEM) revealed that the presence of water has a vital role on the dispersion of clay in the fluid while presence of calcium chloride lowers both the stability of the fluid and the dispersion of the clay. Samples containing calcium chloride exhibited instability and phase separation within a 24-hours period. SEM micrographs and rheological measurements also revealed that the degree of clay dispersion depends on the polarity of the base fluid and the interaction of the polar groups in the fluid molecules with those of clays. The observations on the prepared samples at ambient temperature demonstrated that the minimum amount of clay content for a homogeneous and stable fluid is about 6 lb/bbl. The latter observation also confirmed the role of the organophilic clays on the stability of the invert emulsions.

Investigations on the effect of shear time on the viscosity of the fluids at ambient temperature resulted in a more appropriate thixotropic properties for esters in comparison with olefins and paraffins, e.g. ester based fluids reached the maximum viscosity after 20 minutes of mixing while that was about 60 minutes for the paraffin and olefin based fluids. The latter observation clearly demonstrates the superior dispersion behavior of the clays in esters. The occurrence of maximum viscosities shifted toward lower mixing times at elevated temperatures. This observation shows the favorable influence of the temperature on the thixotropic behavior of the SBFs.

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