The Effect of Shear on Barite Sag

Abstract

Sag of weight material is an oil or geothermal well drilling phenomenon that is difficult to determine from industry standard measurements.  To create 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 responses must be larger than the energy loss responses in the fluid.  This is possible in a water-based drilling fluid, but not in an oil-based fluid.  Oil-based fluids are a combination of a multimodal dispersion and an emulsion.  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 or solid particles.  Adding barite to a drilling fluid will increase the density and also bring more particles with a huge surface area and thus viscosify the fluid.

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 the drilling fluid is sheared properly, both the droplet size and the interdroplet distances are reduced.  The barite particle size has not been changed.  But the barite particle cannot move without creating a flow making a collision between water droplets.  Even though it is not measured on viscosity measurements made in accordance with standards, this increases the viscosity significantly seen from the individual barite particles.  Hence, sag can be reduced by shearing.