Spacers and flushes are effective displacement aids, because they separate incompatible fluids such as cement and drilling fluid. A spacer is a fluid used to separate drilling fluids and cementing slurries. A spacer can be designed for use with either water-based or oil-based drilling fluids, and prepares both pipe and formation for the cementing operation. For example, a spacer is a volume of fluid injected ahead of the cement, but behind the drilling fluid. It can also enhance the removal of gelled drilling fluid, allowing a better cement bond.