Effect of particle number density on rheological properties and barite sag in oil-based drilling fluids

Abstract

The effect of barite particle mass fraction and of particle size distribution on the settling and sag potential in oil-based drilling fluid samples is investigated for four fluids with densities of 1.43, 1.55, and 1.60 specific gravity (S.G). The fourth fluid sample, with density 1.60 S.G., had sieved barite particles of sizes less than 40 μ m . The settling potential in a liquid column under static condition has been characterized using light scattering measurements whereas sag potential in a sheared liquid column has been characterized under a dynamic condition using a rotating cylinder rheometer to produce rotational shear. Extensive rheological characterization of the drilling fluid samples is carried out where the Herschel-Bulkley fluid parameters were defined. We observed that the rate of particle settling under static condition was faster in the fluid sample of S.G = 1.43 having the least barite concentration. There was longer suspension of particles within the fluid sample of S.G = 1.60. Fluid sample with the sieved barite recorded weak destabilization of particles within the middle and bottom column of the sample after 7 days. The viscosity and viscoelastic-linear elasticity also increased with increasing barite concentration which contributed to lower dynamic sag for all shear rates. Time-dependent oscillatory shear measurements provide new insights on the structural character of drilling fluids to predict barite sag tendencies during the fluid design phase. • Barite sag in typical North sea oil-based drilling fluids is investigated. • Fluid viscosity and viscoelastic-linear elasticity increase with increasing barite concentration. • Yield stress values are strongly dependent on barite particle concentration. • The rate of particle migration is much faster in fluid sample with low barite concentration. • Time-dependent oscillatory shear analysis provide valuable information on the breakdown and growth of fluids' microstructures.