Reducing Pressure Drop in Offshore Pipelines by Controlling the Viscosities of Pressurized Emulsions

Abstract

Abstract Viscosities of emulsions can be substantially higher than the viscosity of crude oil especially if the emulsion is tight. High pressure drops could develop in offshore pipeline networks due to this problem. It is shown that by adding a demulsifier offshore the viscosities of the emulsions can be reduced substantially. This paper describes the results of a study that was initiated to determine the viscosities of pressurized crude from an offshore field in Saudi Arabia. The viscosities of the emulsion were measured and determined to be very high. Several tests were conducted to determine the effect of different variables on pressurized emulsion viscosity including temperature, pressure, watercut, and demulsifier dosage. The paper presents some very interesting results for emulsions measured under pressurized conditions (for the first time). The viscosity of the emulsion can be reduced substantially by adding a demulsifier. The demulsifier breaks the emulsions into oil and water thus reducing the mixture viscosity. The results can significantly impact the transportation of crude oil from the offshore field and the required surface/subsurface facilities to produce it. Introduction During the engineering study of pipeline crude transportation from an offshore field in Saudi Arabia it became necessary to determine the viscosity of pressurized oil-water emulsions. The field is located approximately 50 km offshore in the Arabian Gulf and the planned producing facilities consist of a number of wellhead and tie-in platforms that will be connected to the pipeline network transporting oil/gas/water to an onshore processing facility. Production from several wells will be collected and transported to the processing facility onshore. The wells are to be equipped with Electrical Submersible Pumps (ESP). The oil water mixtures from the field behave as tight emulsions with very high viscosities. The high viscosities will impact the design of ESP and pressure drop calculations in the pipeline network. A study was initiated to determine the viscosities of pressurized emulsions from the field for engineering simulations. Viscosity of emulsions can be substantially greater than the viscosity of either the oil or the water1. This is because emulsions show non-Newtonian behavior because of droplet "crowding" or structural viscosity. A fluid is considered non-Newtonian when its viscosity is a function of shear rate. At a certain volume fraction of the water phase (watercut), oilfield emulsions behave as ‘shear-thinning’ or ‘pseudoplastic’ fluids, i.e., as shear rate increases, viscosity decreases. Fig. 1 shows the viscosities of very tight emulsions at 125°F at different watercuts. The viscosity data indicate that the emulsions exhibit Newtonian behavior up to a water content of 40% (this is indicated by constant values of viscosity for all shear rates, or a slope of zero). At watercuts above 40% the slope of the curves deviate from zero indicating non-Newtonian behavior. The non-Newtonian behavior is pseudoplastic or shear-thinning behavior, i.e., viscosity decreases with increasing shear rates. Notice the very high viscosities achieved as the watercut increases up to 80% (compare with viscosities of oil ~20 cP and water <1 cP). Temperature also has a significant effect on emulsion viscosity. An example of the effect of temperature on emulsion viscosity is in Fig. 2. Emulsion viscosity decreases with increasing temperature (note that the data have been plotted on a semi-log scale). At approximately 80% watercut, an interesting phenomenon is observed. Up to a watercut of 80% the emulsion is a water-in-oil emulsion; at 80% the emulsion "inverts" to an oil-in-water emulsion and the water, which was the dispersed phase, now becomes the continuous phase. In this particular case, multiple emulsions (water-in-oil-in-water) were observed up to very high water concentrations (>95%). The viscosity of emulsions depends on several factors2,3:viscosities of oil and watervolume fraction of water dispersed (watercut)droplet size distributiontemperatureshear rateamount of solids present