Effect of Basic Parameters on Viscosity in Synthetic-Based Drilling Fluids

Abstract

Abstract 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. Introduction Water based fluids, used in most offshore drilling operations in U.S. waters, consist of water, barite, clay, caustic soda, lignite, lignosulfonates and/or water-soluble polymers. In nonaqueous based fluids, the continuous phase is a liquid hydrocarbon mixture or any other insoluble organic chemical. NABFs also contain barite, clays, emulsifiers, water, calcium chloride, lignite, and lime. Water or a saline brine (usually containing CaCl2), at a concentration of 10 to 35 by weight percent, is dispersed into the hydrocarbon phase to form a water/oil emulsion with water droplets less than 1 µm in diameter [1–3]. This emulsion is called an invert emulsion because water is dispersed in the organic phase, and formation solids that come into contact with the NABF become oil-wet. Nonaqueous based fluids are more expensive than WBFs; however, they may be used in difficult drilling situations where their technical advantages are required. NABFs are used frequently to:Drill troublesome, hydratable shales;Drill deep, hot wells where WBFs might be unstable;Drill salt, anhydrite, gypsum, and mixed salt zones;Drill and core hydrocarbon-bearing formations near the bottom of wells;Drill through hydrogen sulfide (H<sub>2</sub>S) and carbon dioxide (CO<sub>2</sub>) containing formations;Increase lubricity to decrease torque and drag when drilling a directional well;Minimize the likelihood of differential sticking of the drill pipe in the hole;Serve as a packer fluid for corrosion control; andServe as a workover fluid where water might damage the formation.