Where well rates are outside the range of the test separator, operators usually have no choice but to estimate well production using back allocation techniques, which rarely capture the variance in flow rate over time. This was the case of a well in the Thistle field in the UK sector of the North Sea, where high-frequency flow rate was accurately calculated using ESP real-time data, which was instrumental in measuring inflow properties.

The liquid flow-rate calculation was based on the principle that the power absorbed by the pump is equal to that generated by the motor, which provides a linear equation that can be resolved for rate. Water cut (WC) was calculated by measuring the production tubing differential pressure (DP), which provides the average fluid density, which is subsequently converted to a WC. Analytical equations are used throughout the process, ensuring that the physics are respected at all times. This yields greater repeatability and confidence than analogous methods, which are based on correlations and artificial intelligence.

The calculations provided continuous liquid flow-rate and WC trends over the 6-month life of the ESP with a frequency of one calculation per minute with excellent repeatability and resolution. This enabled us to calculate inflow properties such as productivity index (PI) and skin, as well as history matching a reservoir simulation of the drainage area. This was essential to explaining why production dropped from 1500 to 500 B/D over a period of 8 months, which was found to be a lack of pressure support. The fall in production also caused production instability because of severe slugging, which could be remediated by concurrently increasing the tubing head pressure and ESP frequency without changing the drawdown.

In addition to demonstrating the superiority of an analytical virtual flowmeter over correlations or artificial intelligence, the method also shows how the model can be calibrated accurately using an alternative pump reference curve, which is independent of flow rate, as opposed to traditional centrifugal pump curves. Flow-rate independence was indispensable because of