Liquid velocity and pump intake pressure are known to have strong effects on the separation efficiency of gas and liquid which may impact the flow efficiency of downhole pumps. This paper describes a new model to evaluate the effectiveness of this separation. The model correctly predicts the decline in separation efficiency with higher liquid flowrates, which is shown to be in agreement with presented experimental data. A calibration procedure is conducted which shows a descending trend of bubble size when the pump intake pressure is increased. The new model shows good agreement with the experimental data in the bubble flow regime, since it handles the two-phase natural separation problem by combining the single-phase flow field solution with a bubble tracking method. It is the first CFD-based simplified model which incorporates the multiple effects that govern the natural gas-liquid separation process. If a correlation for bubble size is properly obtained, the model will cover the comprehensive effects of operational and geometric conditions and provide an accurate prediction of gas-liquid separation efficiency over the full spectrum of normal bottomhole pump operating conditions. Finally, the single-phase flow field is solved in a 2D Cartesian coordinate system. The acceleration of the liquid phase, a nature of fluid behaviour in a cylindrical system, is not considered. This is a necessary improvement in the model to properly calculate the slip velocity in the radial direction.