STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

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678 Drilling and Well Completions
Table 4-50
Estimated Requirements for Oil Mud Properties
Mud Weight Plastic Viscosity Yield Point
PP9 CP I bsll00 ft2 Oil-Water Ratio
a-1 0 15-30 5-1 0 65135-75125
10-12 20-40 6-1 4 75125-ao120
12-1 4 25-50 7-1 6 ao120-a5115
14-1 6 30-60 10-1 9 a511 5-aaii 2
16-18 40-80 12-22 aaii 5-921a
Electrical
Stability
200-300
300-400
400-500
500-600
above 600
agitated in a separate tank. Insufficient viscosity can be corrected either by
adding water (pilot testing required) or by treatment with a gellant.
c. There is no general upper limit on drilled solids concentration in oil muds,
such as there is for water-base muds. However, a daily log of solids content
enables the engineer to quickly determine a solids level at which the mud
system performs properly.
d. Water wet solids is a very serious problem; in sever cases, uncontrollable
barite settling may result. If there are any positive signs of water wet solids,
a wetting agent should be added immediately. Tests for water wet solids
should be run daily.
e. The dispersed water phase of an oil-base mud should be maintained in an
alkaline pH range (Le., pH above 7). Temperature stability as well as
emulsion stability depends upon the proper alkalinity maintenance. If the
concentration of lime is too low, the solubility of the emulsifier changes
and the emulsion loses its stability. On the other hand, overtreatment with
lime results in water wetting problems. Therefore, the daily lime maintenance
has to be established and controlled by alkalinity testing. The recommended
range of lime content for oil-base muds is from 2 to 4 lb/bbl.
f. CaCl, content should be checked daily and corrected.
g. The oil-water ratio influences viscosity and HT-HP (high-temperaturehigh-pressure)
filtration of the oil-base mud. Retort analysis is used to
detect any change in the oil-water ratio, giving the engineer a method for
controlling the viscosity of the liquid phase by maintaining a relatively
constant oil-water ratio.
h. Electrical stability is a measure of how well the water is emulsified in the
continuous oil phase. Since many factors affect the electrical stability of
oil-base muds, the test does not necessarily indicate that a particular oilbase
mud is in good or in poor condition. For this reason, values are
relative to the system for which they are being recorded. Stability measurements
should be made routinely, and the values recorded and plotted so
that trends may be noted. Any change in electrical stability indicates a
change in the system.
i. HT-HP filtration should exhibit a low filtrate volume (about 3 ml). The
filtrate should be water-free; water in the filtrate indicates a poor emulsion,
probably caused by water wetting of solids.
Gaseous Drilling Mud Systems
The basic gaseous drilling fluids and their characteristics are presented in
Table 4-51.