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Weighted and Un-weighted mud per API RP13C

Weighted mud and un-weighted mud are widely used in oil and gas well drilling. According to API RP13C regulations, they have difference below

Weighted mud

Traditionally, centrifuging is used with weighted fluids to reduce dilution requirements by eliminating the very small-sized drilled solids and barite (colloidal particles).

Comparison of the cost of the centrifuging with the value of the barite recovered from the discarded fluid is a frequently used measure of its economic effectiveness. However, the purpose of solids removal equipment is to eliminate undesirable drilled solids.

Centrifuging has a direct effect on waste volume. The dilution volume of the centrifuge feed and the disposal of the liquid in the overflow increase waste volumes significantly. Less obvious, but of greater importance, is the fact that disposal of the colloids and near-colloids with the liquid provides a better drilling fluid, reduces dilution requirements, and therefore the volume of waste generated. 

Un-Weighted mud

The economics of discarding the underflow of centrifuges used for solids reduction with unweighted drilling fluids can be evaluated by comparing the cost of the solids removal with the cost of the dilution required by the incorporation, rather than removal, of the separated solids, and the differences in waste disposal costs.

The effect of centrifuging upon the drilling fluid cost can be determined by calculating the volume of dilution that would have been required to compensate for the incorporation of the separated solids, and multiplying by the unit cost of the fluid.

All dilution adds directly to waste volume, thus the cost of disposing of the dilution volume needs to be added to the cost of preparing it. 

Weighted mud and un-weighted mud will be processed by centrifuge in different method or configuration. However, the general process for drilling fluids is almost same. From shale shaker, cone separator, centrifuge, and so on

Weighted and Un-weighted mud