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138 Part II Gas Drilling Systems

6.4 GAS INJECTION PRESSURE REQUIREMENTS

Compressors should be selected for gas drilling to deliver adequate gas
pressure at the design gas injection rate. The design gas injection rate should
be determined based on the flow capacity of the candidate compressor. The
design gas injection rate should never be less than the minimum gas injec-
tion rate required by hole cleaning. With the design gas injection rate, the
required compressor pressure can be calculated on the basis of the extreme
borehole architecture and the extreme borehole conditions.
The extreme borehole architecture is usually the well configuration at
the total depth to drill with gas. Well trajectory can consist of straight,
deviated, or horizontal sections. The drill string configuration includes the
drill pipe, the bottomhole assembly (BHA) with or without a gas hammer,
and the drill bit. The extreme borehole condition is usually the situation
where formation water is encountered and unstable foam is used for water
removal. This requires knowing the intensity of the formation water influx.
The procedure for determining the required gas injection pressure is
as follows:
1. Predict the minimum gas injection rate based on the hole cleaning
requirement.
2. Find a compressor that has a gas capacity higher than the minimum
required gas injection rate with a flow rate safety factor of not less
than 1.15 or a kinetic energy index of not less than 1.2.
3. Use the gas capacity of the compressor as the design gas injection rate
to calculate the gas injection pressure in the extreme borehole
architecture under the extreme borehole condition—specifically:
• Calculate the flowing pressure at the kickoff point in the annulus
using Eq. (6.18).
• Calculate the flowing pressure at the end of the angle-building
section in the annulus using Eq. (6.29) or repeatedly using Eq. (6.26).
• Calculate the flowing pressure at the end of the angle-holding
section in the annulus using Eq. (6.26). If the angle-holding
section is horizontal, use Eq. (6.28).
• Calculate the flowing pressure above the drill bit inside the BHA
by numerically solving Eq. (6.30) for the upstream pressure.
• Calculate the flowing pressure at the top of the drill color inside the
BHA using Eq. (6.26). If the BHA is horizontal, use Eq. (6.28).