BAKER HUGHES - Drilling Fluids Reference Manual

Contamination of Water Based Fluids
Excess lime, lb m /bbl = 0.26 (P m – F w P f )
Pretreating can present a problem, since it is difficult to predict the extent of contamination prior
to drilling the cement. Overtreating with bicarbonate of soda or SAPP could be as detrimental to
drilling fluid properties as cement contamination. Therefore, it is not advisable to pretreat with
more than 0.5 to 0.75 lb m /bbl of bicarbonate of soda. Materials such as UNI-CAL ® and LIGCO
are also suitable pretreating agents because they buffer the pH and aid in deflocculating the
system.
Consideration also should be given to low-gravity solids content prior to drilling cement since
high clay solids content is a primary cause of flocculation when cement contamination occurs.
This could be particularly evident in lightly treated or under-treated fluids or high-density fluids.
Reduction of low specific gravity solids, if high, is recommended as a defense against severe
flocculation.
Because pH values are high when drilling cement, the quantity of calcium ion in solution may not
exceed 200 to 400 mg/L. For this reason, much of the cement drilled remains as discrete particles
and is available to replace the calcium that has been treated out of solution. High concentrations
of excess lime may require many days to remove, particularly if mechanical solids removal
devices, i.e., fine screen shakers, hydrocyclones, or centrifuges are not used.
If cement contamination occurs immediately prior to well completion, it may be necessary to
replace the fluid in the hole with an uncontaminated drilling fluid to serve as a packer fluid,
should testing indicate a high temperature gellation problem.
One approach to avoid over-treatment with bicarbonate of soda or SAPP is to treat only the
soluble calcium ion on a circulation basis and treat the lime particles (cement) that subsequently
go into solution on additional circulations. Generally, treatments should be discontinued when
excess lime approaches the 0.3 to 0.5 lb m /bbl range.
Sometimes, cement contamination is simply allowed to work its way out of the fluid over several
days without “extra” chemical treatment. A well treated fluid could give enough protection
against cement contamination and the often observed rheology and filtration control problems.
Higher Ca ++ measurements, relatively high pH values, and relatively higher P m measurements
could be carried for some time without rheology or filtration control problems with a well-treated
fluid.
Chemical removal of calcium ion with bicarbonate prevents further contamination, but may not
eliminate the rheological problems caused to a dispersed system. It is usually necessary to treat
with deflocculants, such as UNI-CAL, UNI-CAL CF, DESCO, or MIL-TEMP ® (or ALL-
TEMP ® ) to obtain the desired rheological properties. Materials, such as CHEMTROL ® X,
LIGCO, prehydrated MILGEL ® slurries containing UNI-CAL, CMC, or MIL-PAC TM LV could
be used to bring filtration and filter cake characteristics back to the required levels.
A significant problem that may occur as a result of cement contamination is high-temperature
solidification. Since testing at ambient temperature might not identify this problem, tests which
simulate downhole conditions should be run. The FANN Consistometer, FANN Model 70, and
Chandler 7600 XHPHT Mud Viscometer simulate downhole temperature, shear, and pressure
conditions, and should be used to support wellsite testing. These tests give an indication of
solidification tendencies. To determine remedial treatments, it is recommended that pilot testing
procedures include hot rolling and static aging testing when possible. Hot rolling and static aging
testing more accurately simulate reactions which occur in the drilling fluid at elevated
temperatures.
Baker Hughes Drilling Fluids
Reference Manual
4-4 Revised 2006