Caution: When working with NAFs, remember that the viscosity of the base fluid is very temperature sensitive and that viscosity is one of the primary factors influencing sedimentation. The effectiveness of centrifuges is very significantly reduced with high-viscosity fluids. As with other solids-separation devices, it is imperative that centrifuge use be monitored. The volume and composition of the discharge stream should be checked daily to determine the approximate volume of highand low-gravity solids being separated. Occasional particle size analyses should be run on feed, underflow, and overflow. (See Chapter 14 on Capture Equations for additional information on monitoring separator performance.)
Centrifuging Hydrocyclone Underflows
Decanter Centrifuges can be used to recover fluid from hydrocyclone underflows, thereby reducing drilling waste volume. The process returns the finest solids to the mud system, which can present problems. It is important that the solids content of the recovered fluid be monitored. As long as it is less than twice the desired solids content, the results are beneficial. At higher concentrations, it increases dilution requirements, making the process counterproductive.
With unweighted water-based fluids, chemical flocculation can be used to cause colloidal and ultra-fine particles to form aggregates large enough to settle and to be separated by centrifuges. This technique has been used to recover and permit the reuse of water from discarded drilling fluid. Care must be taken to ensure that the residual chemicals from the treatment do not upset the drilling fluid chemistry when returned to the active system.