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Chapter 8: Air, Gas, and Unstable Foam Drilling 8-21
pressures up to 1,000 psig and intermittent operation at pressures up to 1,200 psig.
These high pressures are usually not encountered in air and gas drilling operations.
But sometimes unforeseen high back pressures are encountered while drilling (e.g.,
high influx of formation water or producing fluids) and such pressures have to be
managed in order not to lose the well. Also, if more volumetric flow rate is required
(to lift formation fluids) during the drilling operation, a third compressor unit can
placed in service (all deep drilling operations have a standby compressor available).
The addition of this third compressor will also result in higher injection pressures.
8.3.2 Major and Minor Losses and Injection Pressure
Once the compressor system units have been selected for a given drilling
operation, the actual volumetric flow rate capability of the compressor system will
be known. Knowing the actual volumetric flow rate to be injected into the top of
the drill string, calculations can be made to determine the injection pressure required
by the individual compressors in the system. It is important to accurately determine
the injection pressure. Once the injection pressure has been determined, this value is
compared to the maximum output capability of the compressor. If the obtained
injection pressure value is less than the maximum pressure capability of the
compressors, then the compressor system is tentatively capable of providing
compressed air to the drilling operation. To complete the assessment of the
compressor system, the prime mover of the compressor must be checked to ascertain
if it is capable of producing the power needed to produce the required pressure. The
power output of a prime mover is sensitive to its surface elevation location. The
power output of a prime mover will decrease as the surface elevation location above
sea level is increased. Therefore, it is necessary to compare the horsepower needed
by the compressor to the horsepower available by the prime mover at its surface
elevation location.
The calculation procedure for determining the injection pressure requires
working from the exit to the blooey line (i.e., the known atmospheric pressure)
upstream through the circulation system to injection pressure. During this
calculation procedure, all the pressure positions in the annulus and inside the drill
string where there are changes in annulus and drilling string geometry will be
determined. Of particular interest to the oil and natural gas recovery industry and the
geothermal fluids recovery industry is the determination of the bottomhole pressure.
In underbalanced drilling operations it is very important to design the drill string
used to drill the well. A properly designed drill string will yield the bottomhole
pressure lower than the pore pressure in the fluids producing formation. Thus,
accurate pressure prediction in the circulation system is needed to assure that the
drilling operation will not cause formation damage.
In order to calculate accurate circulation pressures (i.e., bottomhole and injection
pressures) it is necessary to utilize all possible standard fluid mechanics major and
minor friction loss terms in carrying out these calculations. Descriptions of these
major and minor losses are detailed in the fluid mechanics literature [12]. These
losses will be explained and utilized in the calculations used in the illustrative
examples that follow.
Illustrative Example 8.3b describes the implementation of the basic planning
step No. 9 given in Section 8.1.