Page 173 - Drilling Technology in Nontechnical Language

164 Drilling Technology in Nontechnical Language Second Edition

Air may be injected at an appropriate rate in proportion to the mud
circulation rate. Generally, the technique is limited to a maximum depth of
about 2,800 ft, as injection pressures become excessive at greater depths.

In a foam mud, the liquid is a continuous phase and contains encapsulated
air bubbles within it. The percentage of liquid will vary between 2% and
15% by volume.
The lifting capacity of stable foam is superior to that of drilling muds;

cuttings are circulated out of the well more efficiently with foam. It is

possible to displace fluids from the hole using foam. Oil and saltwater

influxes are likely to destroy the foam stability, precluding the use of foam
in those conditions.

Designing the Drilling Fluid

In selecting the most suitable type of drilling fluid, many different
factors must be considered. Overall what is required is a mud system
that gives the lowest overall cost of drilling each hole section, except for

through the reservoir. The direct cost of the fluid itself (the cost per barrel
of mud) is but one component of this overall cost. If serious hole problems
occur because the mud was not optimized for the formations in an effort to
“save money,” obviously much more money will be spent than would have
been saved on the mud bill.
When drilling through the reservoir, the key is to minimize damaging
reactions between the mud and the reservoir that lower the production
possible from the well. If a well loses only 10% of its potential production
rate due to avoidable damage from the mud, the cost to the operator in lost

profit over the full life of the well will be large.
Mud cost must be considered but only to choose between technically
suitable systems. Therefore what should happen is that for each hole
system, all technically suitable alternatives should be defined and then the

cost of each can be compared for a final choice.

Physical, rheological, and chemical characteristics can be defined for
each hole section, leading to a list of requirements for the mud system
of choice.

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