Modeling Low-Salinity Waterflooding

Abstract

Summary Low-salinity waterflooding is an emerging enhanced-oil-recovery (EOR) technique in which the salinity of the injected water is controlled to improve oil recovery vs. conventional, higher-salinity waterflooding. Corefloods and single-well chemical-tracer tests have shown that low-salinity waterflooding can improve basic waterflood performance by 5 to 38%. This paper describes a model of low-salinity flooding that can be used to evaluate projects; shows the implications of that model and demonstrates its use to represent corefloods, single-well tests, and field-scale simulations; and gives insight into the reservoir engineering of low-salinity floods. The model represents low-salinity flooding using salinity-dependent oil/water relative permeability functions resulting from wettability change. This is similar to other EOR modeling, and conventional fractional-flow theory can be adapted to describe the process in 1D for secondary and tertiary low-salinity waterflooding. This simple analysis shows that while some degree of connate-water banking occurs, it need not hinder the process. Mixing of injected water with in-situ water delays the attainment of low salinity, potentially preventing attainment of low salinity all together if very small slugs of low-salinity water are used. This paper demonstrates the importance of mixing to modeling of low-salinity flooding and suggests addressing it in engineering calculations. Care must be taken in representing mixing appropriately in interpreting data and in constructing models. The use of numerical dispersion to represent physical dispersion in 1D, radial, and pattern simulations of this process is demonstrated (i.e., coarse-grid simulations are shown to give the same result as fine-grid simulations with an appropriately large physical dispersion). In many applications, the fine-grid simulation necessary to represent appropriate levels of dispersion is not practical, and pseudoization is necessary. We demonstrate that this can be achieved by changing the salinity dependence and shapes of relative permeability curves.