AbstractsEngineering

Generally, drilling fluids may be multicomponent suspensions, emulsions and/or foams, basically consisting of a base fluid (oil or water), viscosifiers/thinners (clays, biopolymers, etc) and weighting agents (salts) designed to lubricate during drilling operation and removal the cuttings, among others functions. In this sense, clays, mainly smectites, and particularly for oil-based drilling fluid, organobentonites are the most typical additives to achieve fluid properties. Accordingly, the knowledge of organobentonite effect on the fluid properties as function of pressure and temperature is essential for designing a fit-for-purpose drilling fluid for any particular wellbore challenge. Consequently, this Thesis focuses on the influence that both pressure and temperature exerts on the rheological and volumetric behaviour of model suspensions, formulated with organobentonites and mineral oils, commonly used as oil-based drilling fluids. Chapter 4.1 deals with the characterization of the non-Newtonian flow behaviour of these rheologically complex materials as function of pressure applying the approach of the mixing rheometry. The results demonstrate that the special geometries used enable to measure the effect of pressure on rheological parameters of drilling fluids, extending the experimental shear-rate window covered by conventional coaxial cylinders to lower values. Chapter 4.2 studies the effect that organobentonite nature and concentration exert on the rheological properties of oil-based suspensions submitted to high pressure, performing viscous flow measurement of model oil-based drilling fluids under high pressures. As expected, the viscous flow behaviour of oil-based fluids is strongly influenced by organobentonite nature and concentration. In addition, the pressure-viscosity behaviour of these oily model suspensions is mainly affected by the piezo-viscous properties of the oil and the properties of the continuous phase. A factorial Sisko-Barus, which combines the shear and pressure influences in the same equation, fits the experimental pressure-viscosity data, fairly well. Chapter 4.3 deals with the rheological characterization as function of temperature and pressure, of two model all¬oil drilling fluids (5% by weight of organobentonite), using both Bingham and Herschel-Bulkley models. These results have been explained on the basis of the compression of the continuous media and thermally-induced changes in effective disperse volume fraction, being the latter attributed to changes in solvency between polymer-covered particles and the oil media with temperature. chapter 4.4 is devoted to study the influence of aqueous phase volume fraction, organobentonite concentration and pressure on rheological properties of model invert oil emulsions (also known as invert-muds). From experimental results, it can be deduced that, at atmospheric pressure, both apparent yield stresses and linear viscoelastic modulus are highly influenced by the aqueous disperse phase volume fraction, as well as organoclay concentration, and high…