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

Abstract

Abstract Oil Based Muds are very complex fluids composed from 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 Oil Based Muds as drilling fluids. In these operations, the mud will experience very low temperature (down to 0°C) and low shear rate when flowing through the riser. It is therefore of primary importance to control muds rheological properties under such conditions. This work presents a rigorous experimental study of Oil Based Mud rheological properties using scientific rheometers. Temperature dependence and low shear rate behavior are specially studied. Due to their complex nature, Oil Based Mud rheological properties will be extremely dependant on the microstructure of the composition. Characterization of this microstructure has therefore been sought during this study. Several Oil Based Muds 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 an Herschell-Bulckley behavior at higher shear rate, the limiting shear rate value between the two regimes being 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 to estimate the size of the emulsion droplets and the clay particles; a simple energetic and structural model is proposed that fit well the data. Finally, a comparison is made between these experiments and those obtained using Fann35 rheometer; concluding that classical Fann35 measurements induces large error for low shear rate rheology. Introduction Originally prepared from produced oil, oil based muds formulations have evolved to very complex compositions of various additives. The base oil may be of various nature, from gasoline to synthetic fluids based on vegetal ester. Additives are very complex: water droplets, surfactants, organophilic clays, viscosifyer, various solids and others. These additives give specific properties to the mud, particularly regarding rheological properties. Use of Oil Based Muds in Deep Offshore environment is common; in these operations, the mud will experience very low temperature (down to 0°C) and low shear rate when flowing through the riser. It is therefore of primary importance to control muds rheological properties under such conditions. This work is an experimental study of the rheological properties of an Oil Base Mud using laboratory rheometer. Influence of temperature and composition is studied and characterization of the mud microstructure is sought.