Page 160 - Air and Gas Drilling Manual

5-2 Air and Gas Drilling Manual
1. Determine the geometry of the borehole section or sections to be drilled
with air or other gases (i.e., openhole diameters, casing inside
diameters, and depths).
2. Determine the geometry of the associated drill strings for the sections to be
drilled with air or other drilling gas (i.e., drill bit size and type, the drill
collar size, drill pipe size and description, and maximum depth).
3. Determine the type of rock formations to be drilled in each section and
estimate the anticipated drilling rate of penetration.
4. Determine the elevation of the drilling site above sea level, the temperature
of the air during the drilling operation, and the approximate geothermal
temperature gradient.
5. Determine whether direct or reverse circulation techniques will be used to
drill the well.
6. Determine the required approximate minimum volumetric flow rate of air or
other drilling gas needed to carry the rock cuttings from the well when
drilling at its maximum depth.
7. Select the contractor compressor(s) that will provide the drilling operation
with a volumetric flow rate of air that is greater than the required minimum
volumetric flow rate (use a factor of safety of at least 1.2).
8. Using the compressor(s) air volumetric flow rate to be injected into the
well, determine the bottomhole and surface injection pressures as a function
of drilling depth (over the interval to be drilled).
9. Determine the maximum power required by the compressor(s) and the
available maximum derated power from the prime mover(s).
10. Determine the approximate volume of fuel required by the compressor(s)
to drill the well.
11. In the event formation water is encountered, determine the approximate
flow rate of “mist” injection water and the water carrying capacity of the gas
flow.
5.2 Direct Circulation
Direct circulation is extensively used in shallow air drilling operations. In
general, direct circulation is used to drill small diameter borehole wells. Reverse
circulation is preferred for large diameter borehole wells.

5.2.1 Minimum Volumetric Flow Rates
In order to initiate the well planning procedure given above, the geometry of the
direct circulation operation must be defined and the anticipated drilling penetration
rate estimated. Figures can be prepared for the approximate minimum volumetric
flow rates for a variety of shallow well and drill string geometric configurations.
The calculations for such figures are carried out using API standard atmospheric
conditions (i.e., 14.696 psia and 60˚F, see Chapter 4). Thus, the figures developed
will give the minimum volumetric flow rate values for air drilling using air at API
standard conditions. Once such figures are developed, the minimum volumetric
flow rates can be calculated for any other atmospheric conditions (surface locations)
from the minimum volumetric flow rates given for API standard conditions. The
minimum volumetric flow rate values are calculated assuming a minimum