Abstract
Oil-based drilling fluids (OBM) are invert emulsion fluids where water droplets are dispersed in the oil phase. Since the fluid normally is mixed on key sites and transported to operations the solid particle phase of the OBM often settles before the fluid is taken into use. To ensure that the fluid has the intended properties before being circulated into the well, reconditioning on site can be required. Since preparing volumes, normally more than 100 m3, of stable emulsion requires both high shear strain and high elongation strain, most shear guns used offshore are not efficient enough. To provide sufficient emulsification to generate a stable fluid, a common practice is to use the down hole drilling bit nozzles to emulsify the mixture after adding fresh drilling fluid to the circulating fluid. This must be done at a slow rate to ensure the stability of the fluid and is therefore a time-consuming process. A compact solution for efficient on-site recondition emulsification of the drilling fluid would therefore reduce time and costs of drilling operations. On-site pump pressure capacity may be limited, and on-site energy use should ideally be kept as low as possible. With these restrictions in mind, it is desirable to be able to provide a stable emulsion with as little pressure drop as possible, while high pump rate is also preferable in order to minimize mixing time. Both criteria favour larger nozzle sizes. We here present results from full-scale laboratory tests with a dual shear gun unit, investigating the effect of different design parameters on the oil-water mixing process and on the stability of the resulting dispersion. Surfactants (emulsifiers) have not been added, to allow fluids to separate and to be re-used after each test, and separation time is used as an indicator of droplet size and thus of the expected fluid stability in a system with emulsifiers. In this paper we present and discuss experimental results from selected nozzle sizes and designs. It is shown that it is easier to create a stable emulsion of a 60/40 oil-water ratio than of a 80/20 system when weight particles are present.