Spacers and flushers in cementing
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. Spacers are typically densified with insoluble-solid weighting agents.
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. Spacers can be designed to serve various needs. For example, weighted spacers can help with well control, and reactive spacers can provide increased benefits for removing drilling fluids. The drilling-fluid/spacer interface, and the spacer/cement-slurry interface must be compatible. The use of the compatibility procedures outlined in API RP10B is highly recommended. Parameters governing the effectiveness of a spacer include:
- Flow rate
- Contact time
- Fluid properties
To achieve maximum drilling-fluid displacement, consider these guidelines:
- Pump the spacer fluid at an optimized rate
- Provide a contact time (10-minute minimum) and volume of spacer that will remove the greatest possible amount of drilling fluid
- Make sure that the viscosity, yield point, and density of the spacer and the cement slurry are at least the same as the drilling fluid
- When an oil-based or synthetic-based drilling fluid is used, the spacer package should be formulated to thoroughly water-wet the surface of the pipe and the formation.
To achieve a high level of water-wettability, test the spacer system using a newly developed API apparent-wettability testing technique. This technique is highly recommended for customizing the spacer/surfactant package to help ensure water-wetting.
Flushes are used to thin and disperse drilling-fluid particles, and are used to separate drilling fluids and cementing slurries. They can be designed for use with either water-based or oil-based drilling fluids. Flushes prepare both the pipe and formation for the cementing operation, and are not typically densified with insoluble-solid weighting agents. They go into turbulent flow at low rates. Flushes are also referred to as washes and preflushes.
- API RP 10B, Recommended Practice for Testing Well Cements, 22nd edition. 1997. Washington, DC: API.