BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
Workover Products
Various materials have been used in an effort to give the desired characteristics to brine workover
fluids while retaining the solids-free benefits. These materials have had varying degrees of success.
Hydroxy Ethyl Cellulose (HEC) is a non-ionic viscosifier widely used in workover applications.
Baker Hughes Drilling Fluids markets this compound under the trade name W.O. 21L. HEC
hydrates in almost all types of brines and is also effective in reducing filtration.
HEC is made by reacting alkali cellulose with ethylene oxide in the presence of isopropyl alcohol.
HEC is made in several viscosity grades depending on the number of side chains and the molecular
weight. HEC is almost completely acid soluble. HEC, like guar gum, is susceptible to bacterial
degradation. The use of an antifoaming agent, such as W.O. TM DEFOAM, may be necessary when
adding HEC to brine solutions.
Xanthan gum, or XAN-PLEX D, is a high molecular-weight natural polysaccharide produced by
the bacterial fermentation of the micro-organism Xanthomonas campestreis. This polymer is
slightly anionic and shows good solubility in sodium chloride, potassium chloride, and to a lesser
extent, calcium chloride brines.
The main advantage of xanthan gum as a viscosifier over guar gum or HEC is that the gel structure
that is formed with xanthan gum in solution is superior, thereby resulting in greater hole cleaning
efficiency. Although xanthan gum is somewhat susceptible to bacterial degradation, a biocide is not
required in most applications.
Guar gum has been used widely as a viscosifier for workover fluids. Guar gum is a non-ionic,
branched-chain, high molecular-weight polysaccharide which is obtained from the seeds of the
guar plant. Guar gum is soluble in almost any type of water except zinc bromide due to its nonionic
nature. Though its physical appearance in solution would indicate that it has near solids-free
characteristics, it too can plug formations in much the same manner as clays.
Recent modifications of guar gum have resulted in a material with a lower residual content (after
acidizing). These derivatives, hydroxypropyl guars, have greatly improved the performance of guar
gum. Guar gum also swells when exposed to certain chemicals such as isopropyl alcohol, which is
used in some sand fixation processes. Guar gum is susceptible to bacterial degradation. Use of a
biocide, such as DRYOCIDE TM , is recommended.
To obtain fluid loss control in a brine workover or completion fluid, it may be necessary to add
solid fluid-loss control agents. The most common additive used is sized calcium carbonate, such as
W.O. TM 30. Calcium carbonate comes in fine, medium, and coarse grades depending on the pore
size or fractures present in the producing zones. Calcium carbonate is acid soluble, thus offering
protection against the plugging of producing formations. Other fluid-loss control agents used in
workover and completion fluids include oil-soluble fluid-loss resins, sized NaCl crystals, and iron
carbonate.
Lost circulation materials (LCM) for workover and completion fluids include calcium carbonate,
oil soluble resins, or fluid loss blends that are normally formulated into pills of 20 to 50 bbl. Baker
Hughes Drilling Fluids markets calcium carbonate as W.O. TM 30 (fine and coarse) and
MIL-CARB TM . All of these materials are normally acid soluble. Fluid loss blends generally
include carbonates, viscosifying polymer, and dispersing agents.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-79