Page 199 - Air and gas Drilling Field Guide 3rd Edition

190 CHAPTER 8 Air, Gas, and Unstable Foam Drilling

pneumatic motors have been tested (drilling with no drill string rotation and with
rotation)[9]. (2) As seen in the illustrative examples that follow, the actual volu-
metric flow rate used in an air drilling operation is determined by the primary
compressor output(s) to be used at the drilling location. If the approximate
minimum volumetric flow rate to the borehole is determined to be 1000 scfm
(472 standard liters/sec) and the compressors to be used to supply the com-
pressed air are rated at 700 scfm (330 standard liters/sec) each, then two com-
pressors will be used to supply the air. Thus, the actual volumetric flow rate to
the well is 1400 scfm (661 standard liters/sec). This gives a factor of safety
(above the minimum required) of 1.4. It is standard engineering practice to use
a volumetric flow rate factor of safety of at least 1.2. This approximate minimum
factor of safety (i.e., 1.2) is also used in determining the actual volumetric flow
rate to be taken from a natural gas pipeline for a gas drilling operation. The use
of primary compressor flow rates above the minimum volumetric flow rate
would result in an operational point for the air and gas system somewhere along
the curve C–D.

8.2.3 Engineering Planning Graphs
In order to initiate the well planning process given earlier, the geometry of the
well must be defined and the anticipated drilling penetration rate estimated.
Figures with graphs can be prepared for the approximate minimum volumetric
flow rates for a variety of deep well and drill string geometric configurations.
These figures are presented in Appendix G. The calculations for these figures
are carried out using API Mechanical Equipment Standards standard atmospheric
2

conditions. These are 14.696 psia (10.134 N/cm abs) and 60 F (15.6 C) (see
Chapter 5). Thus, the figures developed will give the minimum volumetric flow
rate values for air drilling using air at these API standard atmospheric conditions.
Once such figures are developed, the minimum volumetric flow rates can be
determined 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 using a minimum bottom hole kinetic
3 3
energy per unit volume value of 3.0 lb-ft/ft (143.7 N-m/m ). Also, it is assumed
that the drilling is in sedimentary rock formations with an average specific gravity
of 2.7. These deep boreholes figures are developed for a uniform borehole diam-
eter with the top two-thirds of the depth assumed to be cased and the bottom
one-third assumed to be open hole. The Appendix G graphs are similar to the
original curves of Angel except that they include the open hole roughness that
increases the minimum volumetric flow rates by approximately 10 to 13% for
the same borehole and drill pipe diameter combinations as those used in the orig-
inal Angel work. The basic equations used to determine the minimum volumetric
flow rate are derived in Chapter 6.
Illustrative Examples 8.1 and 8.2 describe the implementation of the basic
planning steps Nos. 1 through 7 given in Section 8.1.