BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Time
The calculation of filtrate loss at variable time intervals relative to known filtrate loss and time
interval can be predicted by the following equation:
where,
f 1
T 1
= f × ---------
T
f = known filtrate at a time interval of T
f l = unknown filtrate at a time interval of T 1
For example, if fluid loss is 8.0 mL in 15 minutes, the predicted fluid loss in 30 minutes would be,
f 1 = 8 ---------
30
8
5.48
× = × --------- = 11.3 m L
15 3.87
or if time is doubled, filtrate theoretically would be increased by 41%. It should be noted that the
above equation would apply only under conditions where pressure, filtrate viscosity, and cake
permeability remained constant and no changes in chemical contents occurred due to effect of
temperature and/or flocculation.
Summary
Filtration rate is often the most important property of a drilling fluid, particularly when drilling
permeable formations where the hydrostatic pressure exceeds the formation pressure. Proper control
of filtration can prevent or minimize wall sticking and drag, and in some areas improve borehole
stability. Filtration control poses a question that should be answered only after a thorough study is
made based on past experience, predicted pressure differentials, lithology, formation protection
requirements, and overall economics.
Solids
Quantity, type, and size of suspended solids in a drilling fluid is of primary concern in the control of
rheological and filtration properties. Solids in a drilling fluid are comprised of varying quantities of
weighting materials [MIL-BAR ® , DENSIMIX ® , and/or W.O. 30 (Calcium Carbonate)], commercial
bentonite, drilled solids (sand and shale) and, in some cases, loss of circulation additives. Material
balance equations help differentiate high-specific gravity solids from low-specific gravity solids when
the total solids content is obtained from the retort. These materials balance equations are presented in
Chapter 10, Mechanical Solids Control.
Typically, the only high-specific gravity solid in a drilling fluid is the weight material, MIL-BAR ® ,
ORIMATITA ® or DENSIMIX ®. However, low-specific gravity solids are defined as all other solids
except weight material. Low-gravity solids are comprised primarily of MILGEL ® , drilled solids and,
in some cases, treatment chemicals. In the analysis of low-gravity solids, it will be assumed that any
contribution from treatment chemicals is negligible. Therefore, the analysis of low-gravity solids
distinguishes between the quantity of commercial bentonite added to a drilling fluid and the quantity
of drilled solids incorporated into a drilling fluid.
ADVANTAGE performs solids analysis based on the retort and titration results. If barite is not being
used then the default value for the weight material should be changed to the appropriate density.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 1-25