(354h) Drilling Fluids and Mechanisms of Particle Sedimentation

In order to maintain safe and efficient drilling operations, management of pressure in the wellbore to avoid gas-kick is important. Oil-based drilling fluids serve this task and also aid in transporting cuttings to the surface, cooling and lubricating the bit and drill string, maintaining wellbore stability, and preventing formation damage by creating a filter cake sealing the rock pores [1]. They are non-Newtonian and have a complex formulation, composed of base oil, water, weighting agents, stabilizing agents, organophilic clays and other additives. Since weighting agents (e.g. barite) are particulates that maintain the density of the drilling fluid, their settling ("aka sag") may result in loss of pressure control, uncontrolled flow of fluids from or into the formation and wellbore instabilities. Barite setting is not only a static problem, but is also seen under flow [2]. In this work, we address challenges related to the particle sedimentation in oil base drilling fluids and we try to link the particle carrying capacity of fluids to their rheological properties and microstructure. Thus, we studied the impact of shear and mixing as well as individual components in the drilling fluids on the resulting fluid rheology. A model drilling fluid is defined here, comprised of an oil-water emulsion, a weighting agent, and two stabilizing agents. The rheological measurements of the base emulsion and the emulsion with solid additives, particularly structuring agents such as clays or weighting agents, were collected under different temperature (from 0 to 60 °C) and shear rates (0.01-1000 1/s). Different regimes are observed: a Newtonian behavior at low shear rate and a non-Newtonian (shear thinning) behavior at higher shear rate, and the limiting shear rate value between the two regimes being dependent on temperature. Rheology and sedimentation rate is explored in-operando with gamma densitometry, NMR, and X-ray methods. Since drilling fluids are normally designed with a finite yield stress, depending on the period of standstill, sufficiently small particles are suspended indefinitely. Whereas under dynamic, settling rate increases with shear rate and mixing rate. NMR shows the importance of molecular interactions between the stabilizing agents and weighting agents.