Optimized heavy oil-in-water emulsions for flow in pipelines
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Oilfield operations such as drilling, reservoir management, and production require the injection and/or production of complex fluids to improve the extraction of crude oils. Some of these complex fluids such as drilling muds, fracking fluids, foams, emulsions, surfactants, and polymers, fall under the classification of colloidal suspensions which is one substance of microscopically dispersed insoluble particles suspended throughout another substance. These colloidal suspensions show complex rheological properties that are dependent on the suspension properties, flow conditions, and flow conduit dimensions. Rheology of colloidal suspensions is a complex subject that is still being investigated. The focus of this study is on heavy oil-in-water emulsions. Heavy oil and bitumen resources account for approximately 70% of the remaining oil discovered to date in the world. Heavy crude oils are costly to produce, transport, and refine compared to light crude oils due to the high viscosity of heavy crude oils. To improve the economic viability of producing heavy oils, especially in a time with low crude oil prices, operational expenses must be reduced. One of the main areas to improve is the cost associated with transporting produced heavy oils from production wells to refineries. Currently, heavy oils are diluted with low viscosity diluents such as condensates and light crude oils to lower the mixture viscosity below 350 cSt before heavy oils can be transported through pipelines. The diluted mixtures require up to 50% (vol.) diluents to lower the heavy oil viscosity. High demand and low supply of condensates and constrained pipeline capacities have resulted in pipeline transportation costs of up to $22/bbl of diluted heavy oil from Canada to refineries in the U.S. An alternative method of transporting heavy oils is to transport heavy oils in an emulsified form, heavy oil-in-water emulsions, which can show orders of magnitude lower viscosities compared to the viscosity of heavy oils. In this study, a simple, one-step method of preparing heavy oil-in-water emulsions was developed. The physical properties of heavy oil-in-water emulsions are controlled and modified by optimizing the chemical formulation used to prepare emulsions. Stable heavy oil-in-water emulsions can be prepared with chemical formulations that are tailored to the type of heavy oils and available water sources which can range from freshwater to softened seawater. The rheology of heavy oil-in-water emulsions has been characterized with a rotational viscometer. Heavy oil-in-water emulsions, especially concentrated emulsions, showed complex rheological properties such as shear thinning behavior, two-step yield stresses, two-step wall slips, and rheopexy. A rheological equation and a wall slip equation have been developed to model the rheology of heavy oil-in-water emulsions over a range of shear rates and flow conduit dimensions. Heavy oil-in-water emulsions characterized with capillary tube viscometers showed drastically different viscosity measurements compared to the viscosity measurements obtained with a rotational viscometer. This is important because the flow of emulsions in pipelines are similar to the flow of emulsions in capillary tube viscometers, not rotational viscometers. The lower viscosities measured with capillary tube viscometers are attributed to oil droplet migration away from the tube walls due to the shear heterogeneity observed in Poiseuille (tube) flow. A scaling equation was proposed to relate the viscosity measurements of emulsions with a rotation viscometer to the viscosity measurements of emulsions with capillary tube viscometers. The rheological measurements of heavy oil-in-water emulsions are used to estimate the flow of emulsions in crude oil pipelines with various radii. Viscosity measurements of optimized heavy oil-in-water emulsions with a rotational viscometer showed that heavy oil-in-water emulsions with up to 75% dispersed heavy oil can be successfully transported in crude oil pipelines. Adding the effect of oil droplet migration measured with capillary tube viscometers, heavy oil-in-water emulsions with up to 85-90% dispersed heavy oil can be successfully transported in crude oil pipelines. The cost of chemicals used to prepare 85% heavy oil-in-water emulsion is approximately $1-3/bbl of emulsion. Heavy oil-in-water emulsions also showed drag reduction properties which can significantly increase the maximum flow capacity of crude oil pipelines. Transporting heavy oils as concentrated heavy oil-in-water emulsions appeared to be a competitive if not a better method of lowering heavy oil viscosity compared to the diluent method in terms of cost and flow performance in pipelines.