An Overview of the Fundamental Differences in Performance Between Density-Based and Viscosity-Based Autonomous Inflow Control Technologies

Abstract

Abstract Being part of the lower completion, viscosity-based autonomous inflow control technologies utilize viscosity contrasts to distinguish between wanted and unwanted reservoir fluids. Since the first installation on the Norwegian Troll field in 2008, such technologies have proven extremely successful in preventing excessive inflow of unwanted gas in oil wells. They can also stop water in oil wells, provided that the viscosity contrast between oil and water is sufficient to give a meaningful operational envelope. It is, however, important to fully understand the limitations of viscosity-based technologies when used to control water. In general, the viscosity-based technologies do not depend on viscosity alone, but rather on the Reynolds number, which in turn depends on velocity. In other words, the performance of these technologies is a function of the well's local inflow rate, which will change over time. If the local oil rate is high, the Reynolds number is also high, which may be interpreted as low-viscosity water, inadvertently causing the technology to choke back the oil flow instead. Furthermore, if one device closes inside a well compartment with multiple devices, it may lead to higher oil rate through the other devices and set up a chain reaction, such that they all end up closing or choking, thereby imposing a large pressure drop across the lower completion in that compartment.The Reynolds number does not depend on viscosity and velocity alone, but also on fluid density. For this reason, suppliers of viscosity-based technologies sometimes emphasize that their technology is also density-based. This can be misleading because technologies that depend exclusively on density, do not suffer the same disadvantages. The working principle of density-based technologies is typically insensitive to flow rate, viscosity and Reynolds number, which means that they can stop inflow of water even in light-oil wells with small or non-existing viscosity contrast. At least one of the density-based technologies can close and reopen at any pre-defined phase fraction of unwanted fluid. Furthermore, the density-based technologies allow high flow capacity and low pressure drop in open position combined with high choking efficiency in closed position. This allows choking of watered-out zones without the risk of excessive and premature choking of oil from the same zones.In a constructed case study, the performance of density-based and viscosity-based technologies is compared for a given well compartment with multiple inflow control devices. For the viscosity-based technologies, it is illustrated how the risk of the above-mentioned chain reaction increases with decreasing local productivity, and how the operational envelope of each device might become too limited if the viscosity contrast is small and the geological uncertainty large.