Abstract
Barite sag is a phenomenon that is difficult, or impossible, to determine from standard measurements. It is shown that for creating a sag free fluid it is necessary to create a structure in the fluid even though it is in a liquid state. Thus, the elastic responds must be larger than the energy loss responds in the fluid. This is possible in a water-based drilling fluid, but not in an oil-based fluid. There are no real gels formed in an oil-based drilling fluid. The fluid is thickened by having Brownian motion to develop a geometrical structure of water droplets. Adding barite will increase the density of the drilling fluid. But it will also bring more particle with a huge surface area. Sag in water-based drilling fluids has its highest severity at 1.55 sg. However, because of the nature of polymers in water-based drilling fluids, sag is not a problem as long as the polymers are well dispersed. For higher densities the sagging barite particles float up on the material creating a counterflow helping to disperse the barite. The situation is more complex for oil-based drilling fluids. There are normally no real networks creating a real yield stress or a real gel structure. The viscosity is constructed by adding emulsified water with additives working in the oil-phase between the water droplets. Because of the size of the barite, the oilfield viscometer size is constructed with a gap somewhat larger than 1mm to provide viscosity data relevant for frictional pressure losses in the drill pipe and in the annulus. As mentioned above barite act like a viscosifier in addition to act as a weighting agent. Thus, sag cannot be determined by the properties of the fluid m being much larger than the barite particle volumes. The interesting parts controlling the sag is likely to be due to the droplet emulsion as it forms the fluid volume relevant for the barite. If we now shear the drilling fluid properly, both the droplet size and the interdroplet distanceare reduced. The barite particle size has not been changed. But the barite particle cannot move without making a collision between water droplets. Even though it is not measured on viscosity measurements mad in accordance with standards, this increases the viscosity significantly seen from the individual barite particles. Hence sag is reduced by shearing. In the article these processes are described in detail.