THE DIFFERANCE BETWEEN WEIGHTED AND UNWEIGHTED MUDS

Water as a drilling fluid does not qualify as a mud. If there are no hole (formation) problems that prevent its being the most economical drilling fluid; if neither the geologist, palentologist, nor production supervisor have valid objections; and if it is available, water is seldom if ever surpassed. When the formation requires, or a supervisor demands , filtrate control and / or viscosity and /or gels in the drilling fluid, a “mud” is built. Or if the fluid density required is too high for salt water alone , mud properties are required to suspend barites.

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DECANTING CENTRIFUGES

The decanting centrifuge is the only liquid-solids separation device used on drilling fluids that can remove (decant) all free liquid from the separated solids particles, leaving only adsorbed liquid or “bound liquid ,” on the surface area. This adsorbed liquid is not prone to contain solubles, such as chlorides, nor colloidal suspended solids, such as bentonite . The dissolved and suspended solids are associated with the continuous free liquid phase from which the decanting centrifuge separates the inert solids, and are removed with that liquid. The adsorbed liquid can only be removed from the separated solids by evaporation, which has been neither desirable nor practical so far in drilling mud work.

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Drilling Cuttings Separation

Mineralogy of Cuttings

Drill cuttings are particles of crushed rock produced by the grinding action of the drill bit as it penetrates into the earth. Drill cuttings range in size from clay-sized particles (~ 2 μm) to coarse gravel (> 30 mm) and have an angular configuration. Their chemistry and mineralogy reflect that of the sedimentary strata being penetrated by the drill.

Solids control equipment separating cuttings
Figure 1. Separation of WBM and cuttings is mainly based on particle size and relies on shale shakers, hydrocyclones (mud cleaner), and occasionally a decanting centrifuge. Most cuttings are sand/gravel-sized and are easily recovered on the shale shaker. However, silt- and clay-sized cuttings are difficult to separate from the barite and bentonite of WBM; hydrocyclones and centrifuges may be required.

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