(479f) Microstructure and Rheological Properties of Oil-Based Drilling Fluids

To maintain a safe and efficient drilling operation, controlling of pressure in the wellbore to avoid gas-kick is essential. Drilling fluids serve this task and also assist 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. They are non-Newtonian and have a complex formulation. In this work we have studied oil-based-mud composed of base oil, water, weighting agents, stabilizing agents, organophilic clays and other additives. Since weighting agents (e.g. barite) are particulates that provide the density of the fluid, their settling ("aka sag") results 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 forced by flow. In order to address challenges related to the particle sedimentation in oil base drilling fluids we need to understand rheology and microstructure of these fluids properly. In this study, we tried to link the rheological behavior of oil base drilling fluids to their microstructure. Thus, we studied the rheology as a function of shear rate, temperature, oil/water ratio, an mud composition. We also developed cryological scanning electron microscopy (cryo-SEM) and cryological x-ray spectroscopy (cryo-EDS) methods to image the spatial structure of the mud at nanometer to micrometer scales. Different model drilling fluids are defined here, comprised of an oil-water emulsion, a weighting agent, and stabilizing agents. We examined the response of these fluids to a variety of deformations in order to elucidate how the mechanics of the unyielded material manifest themselves. We also performed measurements over different shear rate ranges, both increasing and decreasing the shear rate in a shear rate ramp with fourteen independent runs with a maximum shear rate of 1000 s^−1, seven with increasing shear rate, and seven with decreasing shear rate, all with different minimum shear rates ranging from 100 to 10^−4 s^−1. All of the data overlap for shear rates slightly above 10^−1 s^−1 and are well fitted by a Herschel–Bulkley model. Oscillatory measurements were carried out at finite strains showing G′ and G′′ as functions of strain for different frequencies (0.1- 10 Hz). The samples are linear up to a strain of about 0.1, after which a strain dependence is observed. Also one can see from the results that the viscosity of the mud is increasing with increasing water content and temperature from 0 to 60 °C.