Dynamic Filtration of Invert-Emulsion Muds

Abstract

Summary Dynamic-filtration experiments conducted on oil-based muds show that the dynamic-filtration rate is much higher than API filtration rates. The use of water-wet solids results in very poor-quality external mudcakes and high fluid-loss rates, Better external mudcakes are formed by mixing equal parts organophilic clay and mud, Filtration-loss-control additives (asphalt mineral pitches) do not reduce the equilibrium filtration rate, but do reduce spurt loss and limit solids invasion. In brine-saturated rocks, the invasion rate for oil-based muds is significantly smaller than for water-based muds because capillary pressure prevents the oil phase from entering the core in oil-based muds. Oil-based mudcakes are softer and more shear-sensitive than water-based mudcakes. Scanning electron microscope (SEM) photomicrographs indicate that oil-based mudcakes consist of individual water droplets coated with clay particles. This cake structure gives rise to the low permeability and shear sensitivity of oil-based muds. Introduction Dynamic filtration of drilling fluids has been studied extensively. Most previous work has focused on filtration of water-based drilling fluids, and only a few studies have addressed dynamic filtration of oil-based or invert-emulsion muds. Ferguson and Klotz showed that the dynamic-filtration rate for oil-based muds is approximately a linear function of shear rate. They reported that invert-emulsion mudcakes have low shear strength, resulting in high-equilibrium filtration rates. Wyant et al. studied the effect of oil-mud additives on dynamic filtration of invert-emulsion muds. Their objective was to correlate the dynamic-filtration rate to the API high-pressure, high-temperature static filtration rate. Their results showed that the long- and short-term dynamic-filtration rates of emulsion muds increase with increasing API fluid loss. They also found that addition of fluid-loss-control agents has a small effect on the long-term (equilibrium) dynamic-filtration rate but reduces the short-term fluid loss rate significantly. From their experimental results. then concluded that an internal rather than an external mudcake is the primary mechanism controlling dynamic filtration of invert-emulsion muds Vaussard et al. indicated that the mudcake formed by invert-emulsion low-solid muds is fragile and easy to erode, even at low shear rates. As a result, spurt loss is very high. Because the mudcake erodes easily, a higher differential pressure results in a higher filtration rate. In this investigation, a specially designed dynamic-filtration cell is used to study the dynamic filtration of 30/70 water/oil invert-emulsion muds with different additives. The experimental results are used to investigate the dynamic-filtration mechanisms of such muds.