BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
well is available at all times. All brines discussed in the previous sections are commonly
used for this application.
Conditioned Drilling Fluids
This category probably represents the largest group used as packer fluids. Conditioning drilling
fluids for use as packer fluids is generally done as a matter of economics rather than performance.
Since an investment has been made in the drilling fluid, why not use it as a packer fluid? In the
early 1960s, lime fluids, for the most part, were replaced with systems like UNI-CAL® and low pH
gypsum fluids. Solidification was less of a problem with these systems, and the industry felt they
could serve as packer fluids if properly conditioned.
The primary advantages of using an existing drilling fluid for a packer fluid are availability and
economics. Considerable expenditures are necessary when a new system is prepared because of
material costs, rig time, and disposal of the existing fluid.
Gelation and formation damage are potential problems when existing drilling fluids are put into
packer fluid service. This is because of the high concentrations of colloidal solids which are
generally present. These solids can be tolerated while drilling since the system can be conditioned
periodically. Under static conditions, however, gelation may progress to the point that the fluid
cannot be pumped when remedial work is required. Also, if cement and/or saltwater enter the
system during workover operations, colloidal solids can flocculate and severely affect rheological
properties.
The inert solids (barite and sand) contained in a drilling fluid are not acid-soluble and may result in
permanent blockage of the production zone. Also, drilling fluids usually are relatively low in
electrolyte content, thus the filtrate could contribute to clay swelling and permeability damage.
Since the well was drilled with the fluid, it is not likely that further surface or near surface damage
to the formation will occur. On the other hand, perforating and fracturing expose undamaged
formations. This work should be done with a solids-free or acid-soluble system, if at all possible.
The initial cost advantages of utilizing an existing drilling fluid as a completion and/or packer fluid
is quite appealing. Long range costs related to drilling fluids (excessive gelation, settling, formation
damage, etc.) should be considered, however, before selecting them as packer fluids.
If existing drilling fluids are to be used as packer fluids, the following steps should be taken prior
to placing them in the hole.
1. Reduce low-gravity solids by centrifuging or diluting to a maximum of 4%.
2. Adjust the pH of the fluid to 11.5 to 12.5 with caustic soda.
3. Add additional deflocculant to compensate for that lost through dilution and
centrifuging.
4. Add SALT WATER GEL ® and/or MILGEL ® to increase viscosity to the desired
level.
5. Treat the fluid with an oxygen scavenger (NOXYGEN TM ) and a biocide. MUD-PAC ®
contains a biocide and corrosion inhibitor suitable for this purpose.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-102