Evensen invented the Ensemble Kalman Filter (EnKF) algorithm in 1994 when he was a researcher at the Nansen Center. The method was originally developed for use in weather and sea forecasting. EnKF-based methods also proved to function extremely well within reservoir modelling and simulation in the petroleum industry, and are now used extensively across the globe.

Using automation and rapid and continuous updates of the reservoir models, ensemble-based methods have resulted in better understanding of the subsurface. Updated and correct information provides oil companies with a better basis for making decisions before they drill new wells or implement various IOR measures. According to the jury, the method can also yield substantial cost reductions, in part through reducing the number of wells.
In the 2000s, Geir Evensen led the work of implementing EnKF in Equinor’s Fast Model Update tool.

Evensen is now chief scientist at the Norwegian Research Centre NORCE, where his tasks include leading a Petromaks 2 project for further development of ensemble methods in petroleum applications. He also works with adapting modelling methods for new areas, such as the health sector and climate forecasting.