Non aqueous vs aqueous overflush scale inhibitor squeeze treatment in an oilfield offshore Norway
Abstract The field under study is located offshore Norway. Due to the need for pressure support, it is anticipated that seawater will be injected and continuous gas lift will be used in a number of wells. Barium sulphate scale deposition is expected as high concentrations of barium have been measured in the formation brine. Scale inhibitor squeeze treatments will form an important part of the scale mitigation plan. Squeeze treatments entail the injection of an inhibitor chemical to prevent scale deposition, the treatment generally consists of the following stages: preflush, main treatment, overflush and shut in. The preflush stage is normally injected to condition the formation, with typically a mutual solvent being deployed to improve inhibitor retention and well clean up times. The chemical slug is injected in the main treatment stage, generally as an aqueous phase. The overflush stage is deployed to displace the chemical slug deeper into the reservoir and thus expose the chemical to a greater surface area of rock to achieve a higher level of retention. Commonly, the overflush is deployed as an aqueous phase; however, it is not always feasible to inject large volumes of water in wells which are water sensitive or which already require artificial lift. Water is denser than hydrocarbons, and therefore more difficult to lift. In these circumstances, a non aqueous overflush, generally marine diesel, may be preferable. The diesel volumes required are feasible for scale squeezes during the first years, although some additional logistic effort and costs are to be considered. The objective of this paper is to compare squeeze treatment lifetime achieved by conventional aqueous and non conventional squeeze treatments, where non conventional refers to treatments where the overflush is split into aqueous and non aqueous stages, typically diesel being used for the non aqueous stage. The simulation and optimisation calculations were performed using a specialised near wellbore model for scale treatments, where a two phase flow model was used to describe the displacement process during the multi stage overflush. Splitting the overflush was found to reduce the squeeze lifetime marginally, as the non aqueous overflush is not as effective as a purely aqueous overflush in propagating scale inhibitor deeper into the formation. However, this is counterbalanced by the fact that a smaller volume of water needs to be injected in the formation, and so reducing the risk of formation damage and most important for this particular case, a smaller volume of water will need to be lifted, so the well may be set back to production with ease.