BAKER HUGHES - Drilling Fluids Reference Manual

Oil / Synthetic Fluids
the area of pipe in contact with the cake, the pressure holding it to the cake, and the coefficient of
friction between the pipe and cake.
Spotting with either BLACK MAGIC ® ‚ or S.F.T. in the zone of sticking is the most successful means
of freeing differentially stuck pipe. The asphalt in these systems seals off the water fluid-filter cake,
which then dehydrates, dries, shrinks, and cracks. Once cracked, the oil-base fluid penetrates behind
the stuck pipe and equalizes the pressure, thus freeing the pipe.
High-Temperature Drilling
Wells where temperatures are above 400°F (204°C) commonly use oil-base fluids. The
solids/chemical tolerance of oil-base fluids is much higher than that of most water-base fluids. Also
there is less oxidation reduction taking place in an oil-base fluid as the oxygen content is nil compared
to that of a water base system. Consequently, certain chemicals/solids are not as highly affected by
temperature.
Wellbore Stability
Soft shales can be controlled or hardened by exposure to oil-base fluids which have high-salinity water
in the emulsified phase. Transfer of water from the shale to the oil-base fluid is attributed to osmotic
forces across the semi-permeable membrane around the emulsified water droplets.
Corrosive Gases
Hydrogen sulfide and carbon dioxide bearing zones, which are hazardous to metal goods and
damaging to water-base fluid rheology, are better controlled in an oil environment. The oil coating
protects the metal from corrosion, and lime or scavengers can be added to control gases, if necessary.
Salt and Contaminating Formations
Several unsuccessful attempts using water-base fluid were made to drill through the Louann salt
formation in North Louisiana. At temperatures above 250°F (121°C) and depths below 12,000 ft
(3658 m), plastic flow of the salt was encountered in several wells. High-weight oil-base fluids were
used to overcome these problems. In the North Sea, oil-base fluids have been used successfully to
drill the Carnallite salt sections. The KCl and MgCl 2 salt sections are not washed out when drilled
with an oil-base fluid.
Differential Sticking
Earlier, the application of an oil-base fluid for freeing stuck pipe was discussed. The chance of
becoming differentially stuck while drilling with an oil-base fluid system is minimal. The oil filtrate,
lubricity, low fluid loss, and the cake are ideal for preventing wall sticking.
Drilling Directional Holes
Oil-base fluid is an ideal fluid for directional work because of its thin filter cake, which gives it the
ability to maintain a gauge hole and reduce torque.
Packer Fluids
Oil-base fluids are used as packer fluids because they serve as a stable non-corrosive barrier against
corrosive formation fluids. In California, a well pack was developed which has a grease-like
consistency that prevents transmission of the shock of earth movement to the casing. In Alaska, oilbase
fluid casing packs are utilized between the strings of casing that pass through the permafrost
zone. This provides insulation to the frozen ground from the warm oil being produced.
Baker Hughes Drilling Fluids
Reference Manual
5-6 Revised 2006