Particle settling in non-Newtonian drilling fluids

Abstract

Particle settling is relevant for several aspects of drilling and completion operations, and is directly related to safety and operational efficiency. The primary function of particles added to drilling fluids is to provide density stabilizing the wellbore and hinder influx of fluids and gas, causing a kick situation. Keeping the particles suspended in the fluids is also critical to avoid problems such as stuck down hole equipment, poor cementing of casings, lost circulation and avoid formation damage. The main objective of the present work is to improve the understanding of particle settling in non-Newtonian drilling fluids. The work focuses on identifying parameters critical for particle settling which are directly relevant for everyday handling and use of drilling and completion fluids. To identify these parameters, laboratory techniques have been developed that allow continuous monitoring of the particle settling process in a fluid. The majority of the parameters identified have previously not been studied by the oil industry. This includes parameters such as the composition of the internal brine phase of oil based drilling fluids, added shear energy during preparation of these and the effect of particle morphology on settling. Operational parameters, such as vibrations during drilling, and the efficiency of solids removal equipment have also been discovered to have significant impact on the particle settling rate. Throughout this work the complexity of particle settling in the fluids has clearly been demonstrated. Conventional settling models have shown not to predict the effect of this phenomena accurately. The complexity of the sag phenomena requires the development of significantly improved practical equipment to optimize and monitor the sag stability of drilling fluids at the rig site.