Page 268 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 244

[ ] Well Programming
2.5.11

depend on the emulsifier package and the type of organophylic clay in
use. Rheology modifiers are often used to avoid or minimize the prob-
lems of barite sag in oil muds. Their function is to provide as much
thixotrophy as possible in a low rheology.
Oil-wetting agents. As well as emulsifiers that are present in invert
emulsion muds, in some cases specific oil-wetting agents are used to
ensure that drilled solids and barite are oil wet. Emulsifiers that are
currently in use in oil mud systems are capable of oil-wetting barite
and drilled solids. Specific oil-wetting agents are not usually required
as compared with earlier use of oil muds. If an oil-wetting agent is
added to an invert emulsion system with substantial barite concentra-
tion, a noticeable decrease in plastic viscosity can sometimes be
achieved. This is due to a decrease in interfacial tension.
An example of an oil-wetting agent used in an oil mud is lecithin.
This comprises a negatively charged phosphate group and a quaternary
amine that is positively charged. The phosphate group will be dissolved
in the oil phase and the quaternary amine is attracted to metal or min-
eral surfaces that are normally negatively charged.
As previously described in “Viscosifiers” and “Filtration control
additives” within this section, oil-wetting agents are processed with
clays and lignites to form organophylic clays or organophylic lignites.
In some cases, the use of oil-wetting agents is specified to be avoid-
ed to ensure that oil wetting of a reservoir sand is prevented. Oil wet-
ting of sands, which is theoretically possible with an invert emulsion
oil mud, is usually limited because of the low amount of filtrate (oil
and some emulsifier but not water unless the emulsion is weak) that
enters into the formation. The zone of damage is usually very low. (See
Section 2.5.11, “Formation Damage with Oil Muds.”)
Bridging agents. Materials such as calcium carbonate bridging
agents can be added to the oil mud to prevent whole mud invasion of
the formation. In some cases against sands with some degree of per-
meability, better return permeabilities have been achieved with bridg-
ing agent present in the mud.

2.5.11. Formation Damage with Oil Muds

Concern has often been shown regarding possible formation dam-
age that may be caused by oil-based muds. In some cases, this concern

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