Thermodynamic Analysis of a Modified Autonomous Flow Control Device for SAGD Sub-Cool Management

Abstract

This paper describes the application of Autonomous Inflow Control Devices to manage sub-cool in SAGD bitumen production wells. A modification of the AICD is proposed to increase the effectiveness of its performance restricting premature steam breakthrough. Typical nozzle, short tube, and hybrid type passive ICD's are good at creating uniform inflow by creating additional pressure drop and flow restriction along the length of the wellbore, but once the steam/water interface reaches the device, the steam preferentially flow through at a higher rate with less pressure drop, creating further pressure drop in the steam chamber surrounding the device, more steam flashing, and a runaway situation which can only be cured by increasing the back pressure on the whole well. The Autonomous Inflow Control Device (AICD) is an active flow control device that delivers a variable flow restriction in response to the properties of the fluid flowing through it. The RCP type of autonomous inflow control device (AICD) is much more effective in restricting vapour phased flow compared to liquid phased flow. The flow resistance of the RCP AICD is dictated by the viscosity and density of the fluid entering the device, not by what exits. As such, it does not create maximum flow resistance until steam reaches it, and for the SAGD process, this is too low a sub-cool value. By adding a small pressure drop upstream of the AICD, the device can respond to the onset of steam flashing, creating a significant pressure restriction and maintaining the desired sub-cool. To ensure the proper design of the device and demonstrate its effectiveness in controlling steam flashing, the flow performance must be modelled honouring the thermodynamics of the associated phase changes. The performance of the modified AICD is analysed in its ability to produce bitumen and water with minimum pressure drop while providing maximum control of wellbore steam flashing. Thermodynamic modelling demonstrates the ability of the modified AICD to reduce the impact of downstream pressure on the maintenance of desired sub-cool, allowing all zones in the SAGD wellbore to be produced at optimum drawdown and maximum rates. The study examines the performance of the device at a variety of flow rates, fluid types, and water conditions upstream of the device, from 10 C sub-cool to 10% steam quality, and compares it to the performance of traditional flow control devices. The modified AICD provides a new and effective approach to managing SAGD sub-cool and maximizing bitumen productivity from a SAGD well, and ultimately improving steam chamber conformance and SAGD economics.