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.
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.
Stoke’s Law, which gives the mathematical relationship of the factors governing the settling velocity of spheres in a liquid stated in its simplest form as:
JB = dD² (Db – Dl) G / 18μ
Anyone familiar with drill site operations is aware that many active mud systems tend to increase in surface volume. The excess must be disposed of by some means after being hauled away, or jetted out, etc. Examine Figure 1 with the following: