STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Muds and Completion Fluids 675
Oil-Base Mud Systems
Oil-base muds are composed of oil as the continuous phase, water as the
dispersed phase, emulsifiers, wetting agents, and gellants. There are other
chemicals used for oil-base mud treatment such as degellants, filtrate reducers,
weighting agents, etc.
The oil for an oil-base mud can be diesel oil, kerosene, fuel oil, selected crude
oil, or mineral oil. There are several requirements for the oil: (1) API gravity =
36” - 37”, (2) flash point = 180°F or above, (3) fire point = 200°F or above, and
(4) aniline point = 140°F or above. Emulsifiers are more important in oil-base
mud than in water-base mud because contamination on the drilling rig is very
likely, and it is very detrimental to oil mud. Thinners, on the other hand, are
far more important in water-base mud than in oil-base mud; oil is dielectric, so
there are no interparticle electric forces to be nullified.
The water phase of oil-base mud can be freshwater, or various solutions of
calcium chloride (CaCl,) or sodium chloride (NaCl). The concentration and
composition of the water phase in oil-base mud determines its ability to solve
the hydratable shale problem. Oil-base muds containing freshwater are very
effective in most water-sensitive shales. The external phase of oil-base mud is
oil and does not allow the water to contact the formation; the shales are thereby
prevented from becoming water wet and dispersing into the mud or caving into
the hole.
The stability of an emulsion mud is an important factor that has to be closely
monitored while drilling. Poor stability results in coalescence of the dispersed
phase, and the emulsion will separate into two distinct layers. Presence of oil
in the emulsion mud filtrate is an indication of emulsion instability.
The advantages of drilling with emulsion muds rather than with water-base
muds are (1) higher drilling rate, (2) reduction in drill pipe torque and drag,
(3) less bit balling, and (4) reduction in differential sticking.
Oil-base muds are expensive and should be used when conditions justify their
application. It is more economic to use oil base mud.
a. to drill troublesome shales that swell (hydrate) and disperse (slough) in
water base muds,
b. to drill deep, high temperature holes in which water base muds solidify,
c. to drill water soluble formations such as salt, anhydride, carnallite, and
potash zones,
d. to drill in producing zones.
For additional applications, oil muds can be used
a. as a completion and workover fluid,
b. as a spotting fluid to relieve stuck pipe,
c. as a packer fluid or a casing pack fluid.
There is one shale problem, however, that can be solved only by an oil-base
mud with a CaCl, water solution. This shale problem is the “gumbo” or plastic
flowing shale encountered in offshore Louisiana, the Oregon coast, Wyoming,
and the Sahara desert. While drilling “gumbo” with water-base mud, the shale
dispersion rate in the mud is so high that the drilling rate has to be slowed
down or the mud will plug the annulus. AI1 solids control problems are encountered,
such as bit balling, collar balling, stuck pipe, shaker screens plugging,
etc. An oil-base mud with a freshwater phase does not solve this problem, but