BAKER HUGHES - Drilling Fluids Reference Manual

Hydraulics
Figure 1-14
Gel Strength Characteristics vs. Time
Filtration
Two types of filtration are considered in this section, static and dynamic. Static filtration occurs when
the fluid is not in motion in the hole. Dynamic filtration occurs when the drilling fluid is being
circulated.
Dynamic filtration differs from static filtration in that drilling fluid velocity tends to erode the wall
cake even as it is being deposited on permeable formations. As the rate of erosion equals the rate of
build up of the wall cake, equilibrium is established. In static filtration, the wall cake will continue to
be deposited on the borehole.
Testing Equipment
The standard API low-pressure filter press consists of a cylindrical cell three inches in I.D. and five
inches high to contain the fluid. The bottom of the cell is fitted with a sheet of Whatman No. 50 filter
paper. Pressure is applied to the top of the cell at 100 psi. The filtrate is collected over a period of 30
minutes and recorded in cubic centimeters (to 0.1 cubic centimeters) as the API filtrate.
The high temperature/high pressure (HT/HP) test is run at a temperature greater than ambient and a
differential pressure of 500 psi for 30 minutes. The filtrate volume collected is doubled to correct it to
the filter area of the API filtration test. The permeable medium used is the same as that used for the
low temperature test. The filter cake should also be checked for thickness and consistency after the
filtrate loss has been tested.
Correlation between API standard fluid loss at 100 psi and ambient temperature and hightemperature/high-pressure
test at 500 psi and 300°F depends on several factors. Cake
compressibility and thermal stability of additives contained in a fluid are primary factors.
Generally speaking, a well treated lignosulfonate / lignite / bentonite system may have a ratio
between HT/HP and standard API filtrate test in the range of 2:1 to 4:1, whereas a system
comprised of a high concentration of drilled solids may have a ratio of 10:1 or higher. Obviously, a
Reference Manual
Baker Hughes Drilling Fluids
1-22 Revised 2006