Successful Cementing Based on New Design Methodology for Displacement of Non-Aqueous Drilling Fluid
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
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.