Page 472 - Air and Gas Drilling Manual

9-76 Air and Gas Drilling Manual
Case History This was a vertical well drilled to 8,130 ft in the San Juan Basin of
New Mexico (Dakota formation target). This was an unusual well in that nearly all
of the borehole was openhole drilling. A 9 5/8 inch surface casing was run from
surface to 380 ft. The openhole was drilled with aerated drilling fluid over most of
the interval from the bottom of the casing to the depth of 8,130 ft. The aerated
drilling fluid was used to reduce drilling fluid loss as the borehole advanced. The
openhole was drilled with an 8 3/4 inch drill bit to a depth of 3,592 ft. At that
depth the drill bit size was changed to a 7 7/8 inch drill bit and drilling continued to
the bottom of the well with this drill bit size. Most of the formations that were
drilled in this long openhole interval were shale sections. The drill string profile
(while drilling at 7,968 ft with the 7 7/8 inch tri-cone roller cutter drill bit) for this
well was 4 1/2 inch drill pipe from surface to 7,356 ft and 6 inch drill collars to
7,968 ft. The incompressible drilling fluid (drilling mud) was approximately 9
lb/gal and was injected into the well at a volumetric flow rate of 250 gal/min. The
drilling gas was inert atmospheric air (specific gravity of approximately 0.97) with
an injection volumetric flow rate to the well of approximately 400 acfm (surface
elevation location of approximately 5,700 ft). Using the major and minor fluid flow
lose calculations as described in this chapter, the surface injection pressure for the
aerated drilling fluid was calculated to be 513 psig. The actual field injection
pressure was approximately 650 psig. The predicted bottomhole pressure was 3,421
psig. No field data were recorded for bottomhole pressures. Although the injection
pressure correlation between predicted and actual is not extremely accurate, the
accuracy is acceptable considering the long openhole section in this well. The longer
the openhole section, the more uncertain the diameter and surface roughness.
The above case history clearly demonstrate the accuracy of the aerated drilling
calculation process. This comparison calculation was carried out using the same
drill pipe body and drill pipe tool joints lumped geometry approximations as those
in Illustrative Example 9.3 above. The success of these calculations depends upon
the careful inclusion of all major and minor fluid flow friction losses.
9.7 Conclusions
The discussion in this chapter has concentrated on direct circulation operations.
It has been tacitly assumed that there are few reverse circulation operations deeper
than the 3,000 ft depth criteria set in this book (see Chapter 5). But it should be
noted that for those rare drilling situations where reverse circulation is used to drill
beyond 3,000 ft, the discussions given above for major and minor fluid flow friction
losses are, in general, applicable to reverse circulation operations.
The demonstration calcuations in this chapter have utilized lumped geometry
approximations for the drill pipe body and drill pipe tool joints. Such
approximations appear to adequately model the overall friction resistance in the
circulation system and give accurate results for bottomhole and injection pressures.
An improvement to this drill string geometry approximation technique can be made
by programming each tool joint indivdually at its proper location in the drill string.
This type of program would be best carried out using a higher level computer
language such as C++ or FORTRAN. Such a programmed solution would improve
the detail pressure versus depth accuracy of the model. However, a comparison of