Before a cement job, cement system compositions are carefully designed in the laboratory at a given anticipated slurry density; however, during the job, significant density errors or fluctuations can occur, and the properties of the resulting cement system may differ dramatically from those observed in the laboratory. For this reason, the accuracy and precision of cement mixing in the field have been topics of concern for many years. In this paper, the design and operation of a newly developed computerized vortex cement mixing system, which provides excellent density control, is described. In addition, case histories are presented which demonstrate the accuracy and presented which demonstrate the accuracy and precision of the mixing system, and the successful precision of the mixing system, and the successful transfer of laboratory cement system performance to the wellsite.
Cement slurry mixing is one of the most important practical cementing problems. The goals of this process practical cementing problems. The goals of this process are to effect a correct proportioning of solids and carrier fluid, and to prepare a slurry with properties equivalent to those expected from prejob laboratory testing. These goals must be met; otherwise, the relevance of the careful job planning is questionable.
During the design phase of a cement job, a program of laboratory tests is usually performed on candidate cement systems to obtain the desired engineering properties. The final result is a cement system with an properties. The final result is a cement system with an optimum thickening time, compressive strength, fluid-loss rate and rheology. The free-water and sedimentation tendencies are also minimized. Test slurries are prepared at the exact densities proposed for the cement job, and one assumes that the slurries will be mixed at the anticipated slurry density during the job. However, as reported by Grant et al., errors are common and slurry properties can be seriously affected by density errors as low as 0.05 g/cm3 (0.4 lb/gal). Grant et al. also argued that, under such circumstances, solid additives are preferable to their liquid counterparts because the concentrations by weight of cement are independent of slurry density, and the resulting slurry properties are less sensitive to density error.
In response to the density-error problem, the industry is currently introducing process-control systems which allow the accurate proportioning of cement system ingredients during the mixing process. Most of the reported techniques involve the retrofitting of microprocessor controlled valves onto common recirculating jet mixers. The present paper concerns a more fundamental departure from conventional mixing technology, whereby the slurry is prepared by a computerized vortex mixing system.
At this point it is essential to mention the existence of other parameters which can affect the slurry properties, including the amount of mixing and shearing properties, including the amount of mixing and shearing energy imparted to the cement system and temperature accuracy.