BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
above 11.0, which can be detrimental to the polymer. In most drilling situations (high salinity
and pH), these polymers will not usually biodegrade. If, however, a polymer system is to be
stored or bacterial degradation is a possibility, a preservative should be added (0.25 to 0.5
lbm/bbl).
Note:
Once fermentation starts, it is too late to treat with a preservative.
Although this system has mostly been associated with low-solids applications, it can be weighted
if higher densities are necessary. The performance of a weighted system can be improved with a
centrifuge and fine screens on the shakers to minimize the incorporation of drilled solids.
Phosphate-Treated Fluids
Phosphates are used for deflocculating drilling fluids. The ones most commonly used are sodium
tetraphosphate (OILFOS ® ) which has a pH near 7.0 and sodium acid pyrophosphate (SAPP)
which has a pH near 4.0.
Sodium tetraphosphate and SAPP are most commonly used to control rheological properties in
freshwater, low-solids systems. Treatments of 0.1 to 0.2 lbm/bbl are normally sufficient when the
system is maintained in the 8.0 to 9.0 pH range. Care should be taken to avoid overtreatments
which results in excessive viscosity.
Since SAPP sequesters calcium and reduces pH, it is often used to combat cement contamination.
The amount required depends upon the severity of the contamination. However, as a general
guide, 0.2 lbm/bbl of SAPP will treat out approximately 200 mg/l of calcium ion. Monitor the
chemical properties of the fluid (P m , pH, P f , Ca ++ ) carefully. When the P m to P f ratio approaches
3:1, discontinue use of SAPP to avoid over treatment.
Phosphates revert to orthophosphates at temperatures above 175°F. Orthophosphates will not
thin above this temperature, but still function as a sequestering agent. Therefore, this material is
rarely used at deeper depths, because of the temperatures normally encountered. Phosphates are
also ineffective as dispersants in the presence of severe calcium and salt contamination.
Under proper conditions phosphate fluids offer good rheological properties at a minimum cost.
Caustic Soda is normally added for pH maintenance. Small quantities of tannin-type dispersants
may further enhance deflocculation of the system. MILGEL ® is added periodically to improve
filter cake and to reduce filtrate.
Tannin and Lignin Fluids
Additives rich in tannin, namely quebracho, were commonly employed as thinners prior to the
advent of lignosulfonate. The material is derived from the quebracho tree and normally has a pH
of 3.8.
Most quebracho products are a quebracho extract blend. These are used primarily as a thinner in
freshwater drilling fluids for moderate depth wells. The product is unstable at temperatures
above 240°F. It also loses its effectiveness in environments containing excess salt (6000 to 7000
mg/L) or calcium ion (240 mg/L). The system can be maintained at a pH as low as 9.0, but it will
tolerate more salt and calcium contamination at pH ranges of 10.5 to 11.5. When used as a thinner
for freshwater fluids, 1 to 2 lbm/bbl of the material is normally required. When used with lime
fluids, 2 to 4 lbm/bbl will be needed for initial conversion. For treatment of cement
contamination in freshwater fluids, concentrations of 1 to 2 lbm/bbl are usually sufficient.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 3-37