Settling of weighting agents in non-Newtonian fluids to off-shore drilling wells: Modeling, parameter estimation and analysis of constitutive equations

Abstract High-density solids such as barite and hematite are generally added to drilling fluids to increase their density, allowing the control of subsurface pressures. However, a variety of problems in drilling wells has been related to the gravitational settling of particulate materials. In this work, the dynamic behavior of barite settling in a synthetic paraffin-based drilling fluid was evaluated experimentally by monitoring the solid concentration as a function of time and height. The tests were conducted in a gravitational settling bench-scale rig using a gamma-ray attenuation technique. Following the standard approach of continuum mechanics, a mathematical model was developed to yield a single set of partial differential equations capable of describing all settling zones inherent in a batch sedimentation process. Further, by means of the predicted solid concentration profiles, it was possible to obtain the height of bottom piled up solids. The mathematical model takes particles compressibility under their own weight and permeability of the sediment into consideration. The parameters of the proposed constitutive equations for the effective pressure on solids and porous matrix permeability were estimated and statistically analyzed from the experimental profiles of solid concentration. In particular, the results obtained in this work indicated a good agreement between the model predictions and the experimental data.