Abstract
This paper summarizes the work of the exploratory phase of a Joint Industry Project (JIP) investigating operational instabilities and ‘no flow’ events observed in a large number of Steam Assisted Gravity Drainage (SAGD) wells produced with ESPs, where, while the ESP is operating, flow to surface suddenly stops. The impact of these events to the SAGD operations has included lost production due to downtime and reduced drawdown, additional stresses to the ESP due to operating without flow for extended periods and repeated shut-downs and restarts and some system failures. The primary objectives of this exploratory phase were to better understand the mechanisms and key factors responsible for these no flow events, to identify possible mitigation actions, which may be in the wellbore trajectory, ESP landing position, ESP system component designs, or operating practices.
This work was structured as a series of progressive, inter-related tasks, using a combination of analytical models and Computational Fluid Dynamics (CFD), to systematically examine the components of an ESP completion and to narrow-in on the primary contributing factors. The approach was adopted in an effort to assess if the instabilities were due to flow conditions upstream of the ESP, past the ESP, through the intake or in the initial stages of the pump. For each step of the analysis, the output boundary conditions of an upstream analysis became the input boundary conditions of the corresponding downstream assessment.
First, a combination of one-dimensional (1D) multiphase flow and multiphase CFD models were used to characterize the fluid conditions and flow behaviour in the lateral and heel of the well below the ESP. Second, flow past the ESP motor was examined using CFD models to examine the impact of motor heating on subcool reduction and steam vapour generation. CFD simulations were also used to examine fluid separation and flow into a bottom feeder ESP intake to assess the amount of non-condensable gas (NCG) entering the ESP and if the pressure drop through the intake was sufficient to cause significant steam vapour flashing. Finally, a representative SAGD ESP stage was analyzed using both a broad suite of analytical surge models to assess the stability of the pump given the gas-liquid fraction entering the pump, as well as CFD simulations of the rotating stage. The focus of the CFD assessment within the entrance and first stage of the pump was to determine whether vapour flashing was occurring within the ESP stage and the apparent impact on the stage’s ability to generate head.
The results from this exploratory phase of the JIP indicated that vapour flashing within the ESP impeller due to insufficient required Net Positive Suction Head (NPSHr) appeared to be one of the dominant mechanisms causing no flow events, as opposed to NCG or steam entering the pump from the intake. Future work for this JIP includes validation of the CFD results using lab test data, further CFD simulations of other ESP stage designs and at a wider range of operating conditions, and examination of alternate ESP designs that may allow for production at lower subcool and lower NPSHr values.
