Page 319 - Air and Gas Drilling Manual

8-2 Air and Gas Drilling Manual
8.1 Deep Well Drilling 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. In some oil and
natural gas recovery operations it is necessary to drill with a gas that will not
support downhole combustion. This objective has been realized by using natural gas
and using oxygen stripped atmospheric air (inert air) as drilling gases. The use of
natural gas from a pipeline as a drilling gas has been in use since the 1930’s. The
use of inert air is a new technology and has become commercially viable in the last
five years. In this chapter atmospheric air, natural gas, and oxygen stripped
atmospheric air will be used as the example drilling gases.
The basic planning steps for a deep well are as follows:
1. Determine the geometry of the borehole section or sections to be drilled
with air or other gases (i.e., openhole diameters, the casing inside
diameters, 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 that might be encountered.
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.
5. Establish the objective of the air (or other gases) drilling operation:
• To decrease or eliminate formation damage,
• To allow underbalanced drilling operations.
6. Determine whether direct or reverse circulation techniques will be used to
drill the well.
7. Determine the required approximate minimum volumetric flow rate of air
(or other gases) to carry the rock cuttings from the well when drilling at its
maximum depth.
8. 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).
9. 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). Also, 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
volumetric flow rate of “mist” injection water needed to allow formation
water or formation oil to be carried from the well during the drill operation.
12. Determine the approximate volumetric flow rate of formation water or
formation oil that can be carried from the well during the drilling operation
(assuming the injected air is saturated for bottomhole conditions).