This paper describes the development of a two-phase near-wellbore simulator to predict the impact on squeeze lifetime of the overflush fluid type. In the past, a single aqueous-phase model was used for a diesel overflush treatment. The new model enables a more accurate description of the displacement process and the impact of inhibitor transport through the formation and retention on the rock matrix. Field data are presented, with various treatments being considered. In each case, the well was treated with the same aqueous scale inhibitor (SI). The initial squeeze treatments used a diesel overflush. However, subsequent treatments used the same inhibitor but used seawater as the overflush fluid. It is clear from the field returns that the use of seawater rather than marine diesel improved chemical placement and extended treatment life. The theory behind this phenomenon is outlined, so allowing for more accurate treatment designs. The process followed involved first deriving an isotherm using a single-phase squeeze model based on the water overflush treatment. This is the established conventional approach used in many hundreds of cases worldwide. This isotherm was then used to model the same treatment using the new two-phase model, which accounts for saturation changes during the treatment. A good match was achieved using the isotherm, giving confidence that the two models agree for purely aqueous treatments. A diesel overflush treatment was then simulated using the two-phase model and the same isotherm, and again a good match was achieved. However, modeling the diesel overflush treatment in the single-phase model required a different isotherm to achieve the match. This clearly indicates that diesel overflush treatments may be modeled accurately using the two-phase model. Additional sensitivity calculations were performed to investigate the impact of splitting the overflush volume into separate diesel and water stages.