Understanding what olifield drilling mud decanter centrifuges really do with drilling fluids and solids can prevent common mistakes and improve mud quality.
Centrifuging splits the processed fluid into two streams: the overflow , containing most of the liquid and the finest solids, and the underflow (cake or heavy phase), consisting of the coarser solids and the liquid wetting them. "Traditional centrifuging" removes the damaging ultrafine and colloidal particles - both barite and low-gravity solids (LGS) - and most of the liquid when the overflow is discarded, while keeping the larger solids particles (desirable barite and larger LGS) and returning them to the drilling fluid.
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Although solids content, typically measured with a retort, is a useful parameter, the adverse effects of solids on rheology and filtration are a function of their total surface area, not the solids content itself. The smaller the average particle size, the greater the surface area. Unfortunately, the retort tells us nothing about particle size.
Solids in drilling fluids are subjected to conditions that cause a progressive reduction in particle size, with a corresponding increase in surface area. This has serious adverse effects on mud quality even though solids content remains constant. We have all observed how hole conditions deteriorate with time. Much of the deterioration can be attributed to the increasing surface area of the mud solids as the mud "ages" and its solids become finer.
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In addition to reducing the plastic viscosity and dilution requirements, removing the colloidal and ultrafine solids will improve both the penetration rate and the filter cake quality, thereby reducing torque and drag. This improves hole conditions and reduces the risk of sticking.
