BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
15. Gas Stripping
Intrusion of gas into an oil-base fluid can cause the weight material to settle. This is referred to as gas
stripping of barite, whereby the intruding gas changes the wetting characteristics of the barite surface
area. This can be a problem when using oil-base fluids on gas wells.
16. Aqueous Contamination
An influx of water due to surface leaks, rain, or formation has a significant impact on the properties of
the system. Dilution is the only recourse to correct the condition and one that is not inexpensive to
treat.
Oil-base fluid applications
Drilling
The greatest use for oil-base fluids in the period from 1935 to 1950 was for well completion, mainly in
low-pressure or low-permeability zones. When oil-base fluid was used instead of water-base fluid,
higher initial production was usually noted. The all-oil filtrate provided an environment in which
water-sensitive clays did not hydrate. More recently, the advent of relaxed filtrate, low-solids oil-base
fluids, and polycrystalline diamond cutter bits has resulted in exceptional drilling rates and cost
savings. Oil-base fluids allow the drilling of reactive formations more efficiently than water-base
fluids; such is evident in the North Sea.
Perforating
Oil-base fluid will not seal off perforations as may be the tendency when a freshwater or saltwater clay
fluid is used. Bullets and jet charges shot in many water-base systems actually deposit hard solids and
clay behind the shot, thereby reducing the effectiveness of the perforation job.
Work over and Completion
Using oil-base fluid in the open-hole section of deep, high-pressure wells simplifies running liners and
casing. The fluid provides maximum lubrication of the borehole, reduces torque, and reduces the
chances of differential sticking. The procedure aids the operator in the holes where close tolerances
are involved or when landing liners in troublesome holes. The same benefits can be seen when
running tubing and setting packers.
Coring
Maximum recovery is normally achieved because formation clays do not react with oil-base fluids.
The operation is made easier by elimination of core swelling and jamming of the core barrel and
differential sticking.
Stuck Pipe Spots
The majority of stuck pipe cases are caused by differential pressure sticking. Other causes are key
seating, balled-up bits, hydrated clay, disintegrated shale formations, poor hole cleaning and washouts.
Differential sticking can be directly related to low-quality fluid filter cake and/or high pressure
differential between the wellbore and formation.
When the pipe becomes partially embedded in the filter cake, it becomes a part of the cake as though it
were a fluid solid. It is held to the cake by the pressure difference between the well-bore pressure and
the pore pressures in the cake under the pipe. The force required to pull the pipe free is a function of
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 5-5