Page 211 - Air and gas Drilling Field Guide 3rd Edition

202 CHAPTER 8 Air, Gas, and Unstable Foam Drilling

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In Figure 8-8, the bottom hole annulus pressure is 161.7 psia (111.5 N/cm abs)and
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the injection pressure is 215.4.6 psia (148.6 N/cm abs).
The somewhat erratic nature of the plot in Figure 8-8 reflects use of the lumped
geometry method used in the Mathcad calculations. A few sophisticated programs
in Fortran or Cþþ treat each individual tool joint at its location and give a smoother
plot. Most commercial programs, however, use an average outside “drill pipe” diam-
eter, which is the weight average (by surface area) of the outside diameter of the
tool joint with the outside diameter of the pipe body. This style of program will also
give a smoother plot of these circulating system pressures. All of these programs
will give injection and bottom hole pressures that are with 1 to 2% of the lumped
geometry Mathcad calculations shown in Appendix C.
However, certain important aspects of air drilling can be missed using the
weighted average method of treating the drill pipe tool joints. There are very practi-
cal deep drilling infill operations in older gas production fields that require
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sidetrack drilling out of the smaller 5 2 = -in (140 mm)casing. In such wells,
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the sidetrack operation can easily utilize 2 8 = -in (60.3 mm)drill pipes thathave
inappropriate tool joints that are too large. These tool joints can create choke pres-
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sures with the 4 4 = -in (120.7 mm) open hole and the cased hole sections at the
tool joints to actually cause formation damage in the reservoir with static bottom
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hole pressures ashighas900 psia (620.7 N/cm abs). In essence, it is probably
important to retain the tool joint dimensions in some form as either lumped geom-
etry or as the individual tool joints in place in the programs.
Figure 8-9 shows the injection pressure and the bottom hole pressure as drilling
takes place from the casing shoe at 7000 ft (2134 m) to a final depth of 10,000 ft
(3048 m). Assuming that this example is more typical of a performance drilling
operation, then it is clear that the same air volumetric flow rate (compressor pack-
age) can be used to drill the entire 3000 ft (914 m) of open hole. The injection pres-
sure increases with increasing drilling depth, but is only slightly higher when
drilling at 10,000 ft (3048 m) relative to drilling at the top of the open hole section
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(i.e., approximately 28 psi higher, or 19 N/cm higher). This lack of change is some-
what a consequence of the choking effect of the drill bit open orifices. This means
that the compressed air inside the drill string is hydrostatic pressure dominated and
not friction pressure dominated [see Equation (6-1)].
It is important to discuss the time for cuttings to reach the surface. In an air
drilled vertical or near vertical borehole, cuttings generated at the bottom of
the annulus as the drill bit is advanced are broken up into small fragments as
the cuttings collide with the rotating drill string and the borehole wall. This
means that within 1000 ft (305 m) or so from the bottom of the annulus in most
wells, the cuttings are reduced to small fragments. This has been demonstrated
by drilling with downhole air motors (with no rotation) and is discussed in
Chapter 11. Such small fragments are carried very rapidly up the annulus to the
surface and will generally reach the surface in a 10,000 ft (3048 m) hole in
approximately 10 min. This delay time is basically a negligible interval when cor-
relating cuttings geology to formations being drilled.