Page 194 - Air and gas Drilling Field Guide 3rd Edition

8.1 Deep Well Planning 185

8.1 DEEP WELL PLANNING
Deep air and gas drilling operations use a variety of compressed gases as the
drilling fluid. The majority of the operations use compressed air or membrane
generated nitrogen (see Chapter 5). The membrane nitrogen will not support
downhole combustion. Since the 1930s, natural gas from pipelines has been used
as a drilling gas since it will not combust in the typical downhole environment.
Today, natural gas is too expensive to use as a drilling gas. Membrane generated
nitrogen is a relatively new technology and has become commercially viable only
in the past decade. In this chapter, atmospheric air and membrane generated
nitrogen will be used as the example drilling gases.
The basic planning steps for a deep well are as follow:
1. Determine the geometry of the borehole section or sections to be drilled
with air or other gases (i.e., open hole diameters, the casing inside dia-
meters, and depths).
2. Determine the geometry of the associated drill strings for the sections to
be drilled with air or other gases (i.e., drill bit size and type, the drill collar
size, drill pipe size, and maximum depth).
3. Determine the type of rock formations to be drilled and estimate the
anticipated drilling rate of penetration. Also, estimate the quantity and
depth location of any formation water influxes that might be encountered.
4. Determine the elevation of the drilling site above sea level, the anticipated
average operational temperature of the atmospheric air during the drilling
operation, and the approximate geothermal temperature gradient.
5. Establish the objective of the air (or other gases) drilling operation:
n Performance drilling
n Underbalanced drilling
6. Determine the required approximate minimum volumetric flow rate of air
(or other gases) needed to carry the rock cuttings from the well when
drilling at the maximum depth section.
7. Select the contractor air compression package that will be used to
provide for the drilling operation with the needed volumetric flow rate
of air and/or membrane generated nitrogen (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 bottom hole and surface injection pressures as a func-
tion of drilling depth (over the interval to be drilled). Also, determine the
maximum power required by the compressor(s) and the available maxi-
mum derated power from the prime mover(s).
9. Determine the approximate volume of fuel required by the compressor(s)
to drill the well.
10. In the event formation water is encountered, determine the approximate
volumetric flow rate of “mist” injection water needed to allow formation