Modeling a surfactant preflush with non-aqueous and aqueous scale inhibitor squeeze treatments

The most common method for preventing downhole scale formation is by applying a scale inhibitor (SI) squeeze treatment. In this process, a SI solution is injected down a producer well into the near wellbore formation. Commonly, squeeze treatments comprise the following stages: a preflush, the main scale inhibitor pill, an overflush and a shut-in. This is followed by the resumption of production in the well. A surfactant preflush is often injected to "condition" the rock (by altering the wettability, resulting in enhanced SI retention). In these treatments, the main SI pill and overflush fluids may be either aqueous or non-aqueous. This paper presents a near-well radial SI squeeze model that can simulate the impact of a surfactant preflush on both inhibitor return concentrations and on well cleanup time. Specifically, we model the effect of surfactant injection on the reduction of residual water and oil saturations, and the results of these changes on enhanced SI adsorption. These may be considered reversible or irreversible processes. Data from scale inhibitor coreflood tests, in which a surfactant preflush was either included or was not, are used to constrain the model. Then we present a sensitivity study of the impact of the surfactant volume injected, and of the effect of surfactant adsorption onto the rock surface, on enhanced SI adsorption. Of particular interest is that enhanced SI adsorption due to the surfactant showed a significant impact on treatment lifetimes. Finally, an example is presented of modelling treatments in wells in an offshore sandstone reservoir where surfactant preflushes were applied. The modelling results are compared with the field return profiles. The value such a model brings to the interpretation of previously conducted squeeze treatments and to the design of new treatments is discussed in terms of optimisation of chemical cost and treatment lifetimes.