Page 175 - Drilling Technology in Nontechnical Language

166 Drilling Technology in Nontechnical Language Second Edition

the well. (As explained previously, filtrate is the liquid part of the

mud. When mud is forced against a permeable zone, the solids in
the mud form a plaster or “wall cake” against the formation face.
Some of the liquid fraction will filter through this cake and into

the formation. This liquid fraction [water plus dissolved salts] is

called filtrate.)
6. Higher mud densities have higher solids content, and this will
adversely affect mud rheology, possibly calling for more additives
to control this.
On balance, the correct density within this range of maximum and
minimum will normally be closer to the lower limit.
Fluid loss. The fluid loss property of a mud indicates how well the

mud forms a seal against permeable formations. To test fluid loss, a sample

of mud is placed inside a chamber, which has a standard filter at the bottom.

The chamber is closed and 100 psi is exerted on the mud sample. Filtrate
is squeezed through the filter into a container below, and wall cake builds

up on the filter. The standard test measures the amount of filtrate collected

in 30 minutes, with the thickness of the filter cake given in 1/32nds of an

inch or in millimeters. A description of the filter cake might also be made,

using descriptions such as hard, soft, tough, rubbery, firm, etc. Figure 7–5

shows a filter press, which is used to test the fluid loss.

A high fluid loss mud will build up a thicker, stickier wall cake, which

is likely to lead to problems such as pipe sticking in the hole. Ideally the
mud should build up a thin, tough, and impermeable cake fairly quickly.

The test for fluid loss is a comparative test. It does not indicate how

much filtrate will actually be lost to the formation, or how thick the filter
cake might actually become. These things depend on many factors, such
as the actual pressure overbalance, the permeability of the downhole
formation, and the effects of mud flow or pipe movement eroding the

filter cake.

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