Modelling the impact of diesel vs water overflush fluids on scale squeeze treatment lives using a two-phase near-wellbore simulator

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 both brine and hydrocarbon overflush treatments. The new model enables a more accurate description of the displacement process if a hydrocarbon fluid (eg diesel) is used, and the impact on inhibitor transport through the formation and retention onto the rock matrix. Field data is presented, with various treatments where either seawater or diesel are applied as displacement fluids being considered. In each case, the well was treated with the same aqueous scale inhibitor. The initial squeeze treatments used a diesel overflush. However, subsequent treatments utilised the same inhibitor but with 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, modelling 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 accurately modelled 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 to improve well clean up while reducing the logistic challenges associated with pumping large volumes of diesel. Copyright 2008, Society of Petroleum Engineers.