Dispersed Systems in Heavy Crude Oils

Due to very large reserves, heavy crude oils represent an important challenge for the oil industry. Their production has to handle very viscous hydrocarbons and involves the use of advanced methods such as cold production or steam injection. Cold production is faced with the foamy oil phenomenon: while extracted, a heavy crude oil is submitted to a depletion that can induce the formation of dispersed gas bubbles and makes it appear as a foam. With steam injection, water is invariably produced and emulsions may be formed. These two situations show how heavy crude oils are concerned with dispersed systems. The aim of the article is to evaluate the impact of this dispersed phase, either bubbles or droplets, on the flow properties of heavy crude oils. To do so, measurements have been carried out with a controlled stress rheometer under pressure and temperature. Live samples were obtained by recombining a stock tank oil with methane inside the pressure cell of the rheometer. The first part of the article is dedicated to the foamy oil phenomenon. It demonstrates that contrary to spherical bubbles, deformed bubbles can give rise to significant viscosity reduction, which is governed by the capillary number. The second part deals with water-in-oil emulsions. Again, experimental results show that dispersed droplets increase viscosity unless they get sufficiently elongated by the applied shear rate. The last part of the article is particularly original as it studies some rheological experiments of heavy crude oils in presence of both gas bubbles and water droplets. It investigates to what extent the presence of water influences the bubbly characteristics of live heavy crude oils. The experimental work suggests that for optimum design and operations, it is essential to determine oilfield viscosities i.e. under pressure and temperature on live and changing dispersed systems