Influence of Temperature and Clays/Emulsion Microstructure on Oil-Based Mud Low Shear Rate Rheology

Abstract

Summary Oil-based muds (OBMs) are complex fluids composed of water, oil, organophilic clays, and various additives. Their very good filtration and lubricating properties make their use beneficial in numerous drilling operations. Deep offshore operations may present phases using OBMs as drilling fluids. In these operations, the mud will experience extremely low temperature (down to 0°C) and low shear rate when flowing through the riser. It is therefore of primary importance to control mud rheological properties under such conditions. This work presents a rigorous experimental study of OBM rheological properties using scientific rheometers. Temperature dependence and low shear rate behavior are specially studied. Because of their complex nature, OBM rheological properties will be extremely dependent on their microstructure. Characterization of this microstructure has therefore been sought during this study. Several OBM compositions have been investigated. Flow curves have been established with a precise and reproducible procedure at temperatures varying from 0 to 80°C. For an offshore designed composition, a specific behavior at low shear rate has been detected. Two different regimes can be seen: a Newtonian behavior at low shear rate and a Herschel-Bulkley behavior at higher shear rate. The limiting shear rate value between the two regimes is dependent on temperature. Experiments on partial compositions (partial muds with/without solids, with/without clays) have shown that the specific clay used in this composition is responsible for this particular behavior at low shear rate. Oscillatory rheometry measurements have confirmed the existence of these two different regimes. CryoMeb and microscopic observations have allowed us to estimate the size of the emulsion droplets and of the clay particles. A simple energetic and structural model is proposed that fits the data well. Finally, a comparison is made between these experiments and those obtained using a Fann 35 (Fann Instrument Co., Houston) rheometer, concluding that classical Fann 35 measurements induce large error for low-shear-rate rheology.