Drilling fluid removal by spacer fluids is an important step in ensuring proper cementing and therefore adequate zonal isolation, especially when non-aqueous fluids (NAF) are used. In addition to the fluid mechanics aspect of displacement, a tensioactive package containing surfactant(s) and solvent(s) is typically added to the spacer base fluid to clean all surfaces, including the casing. However, current surfactants and solvents on the market have limited application in terms of temperature and type of drilling fluids.

API (10B-2) and ISO (10426-2) recommend practices for evaluating the suitability of a spacer: among others, the viscosity of NAF, spacer, and cement mixtures at various fixed ratios determines their rheological compatibility; reverse-emulsion test or spacer-surfactant screening test (SSST) determines the percentage of spacer for inverting the NAF emulsion. Other non-standardized tests such as bottle tests or grid tests determine the efficiency of a surfactant/solvent aqueous solution to remove NAF from metallic or glass surfaces. But all these tests suffer from lack of reproducibility and limited automation.

Improved alternative laboratory procedures have already been proposed. They include measuring rheology during SSST, which clearly shows the positive impact of early emulsion inversion on viscosity of NAF/spacer mixtures. In addition, proper preparation of metal surfaces clearly improves repeatability of the cleaning test, which now is performed with weighted spacer under temperature and pressure.

Using these improved experimental methods, we performed more than three thousand different tests on more than two hundred tensioactive blends. Statistical analysis helped selecting the optimum chemistry as a function of the conditions (type of base oil, salinity, temperature). This allowed developing guidelines and a tensioactive package comprising a limited number of chemicals (surfactants and solvents), from which the field user would select the ones to combine for his application. Only a few confirmation tests would be necessary at the location for planning a given cement operation.

In addition to the development of the above guidelines and set of chemicals based on the use of the improved laboratory methodology, this paper describes their ease of implementation at field level (few chemicals in inventory to cover all situations and limited local laboratory testing) as well their successful application in several wells (case histories), where different types of non-aqueous fluids were used at various temperatures.