Weighted NAFs present a special problem because of the cost of the base fluid, which can exceed $200/bbl. The objections to series centrifugation raised in the previous section are equally valid for these fluids. Fortunately, NAFs are more solids tolerant than water-based fluids. However, when NAFs are used long enough, ultra-fines and colloids can accumulate to problem levels and force difficult choices.
In most drilling operations, the most expensive variable costs are those associated with rig time. When solids problems cause losses in penetration rate or hole problems that consume rig time and increase the risk of losing the hole and the drill string, a choice must be made between accepting the cost of conditioning the fluid to relieve the problem and accepting the additional expense, and risk, of continuing with a solids contaminated fluid. Aside from dilution, the only means of reducing the colloidal and ultra-fine content of NAF is traditional centrifuging, which removes costly liquid from the system together with the finest solids. Economics clearly favors reconditioning the fluid by removing the colloids so that it can be reused. Unfortunately, centrifuges cannot separate colloids from the base liquid, and no other means of removal has proven to be economical. The advantage of centrifuging, thereby selectively separating the problem solids, over diluting and creating excess volume that must also be separated from the system is that centrifuging permits the desired improvement in mud quality to be achieved at lower cost and with the preparation of less new fluid.
For short-term use in which colloid accumulation is not expected to be
a problem, dilution with fresh, uncontaminated fluid provides a means
of using some of this colloid-laden fluid. It can also be used as packer
fluid or stored for emergency use in the event of lost circulation.