Page 215 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 191

2.4.4
Drill Bit Selection, Parameters, and Hydraulics [ ]

At the time of writing, one of the best evaluation tools is the Dipole
Shear Sonic Imager (DSI) from Schlumberger. This tool gives compres-
sive, shear, and Stoneley wave data, which can be interpreted with
other log data to yield mechanical properties. These results can be dis-
played in a log format. (Note: monopole sonic tools only yield useful
information when sonic velocities are faster in formation than in mud,
i.e., hard rock; dipole tools can be used in softer rock too.) Correct
interpretation of the results takes considerable skill and area knowl-
edge and is therefore not something that drilling engineers would
undertake themselves. In fact, there are many useful interpretations for
drilling purposes that the DSI tool allows and these are covered in more
detail in Section 1.4.7, “Mechanical Properties of Steel.”
Application of the DSI tool to bit selection is in its infancy. Some
general conclusions can be drawn at present, but the complexity of all
the various interrelated factors leaves plenty of room for future research.
There are quite a few papers on these topics (see the listing of references
at the end of this section) which can be followed up for further study.
Following are guidelines that relate various log interpretations to
the basic suitable bit type.

Sonic transit time vs. PDC cutter type. (Courtesy of Smith International Inc.)

µsec/ft 110 100 90 80 70 60 50
3 /4 in (19 mm) cutter ----------------------------------|
3 /4 in (19 mm) dome cutter ----------|
5 /8 in (16 mm) dome cutter ----------------------------------------------|
1 /2 in (13 mm) cutter --------------------------------------------|
1 /2 in (13 mm) dome cutter ------------|
3/8 in (9 mm) dome cutter -----------------------------|
Natural diamond 1-2 stones/carat -----|
Natural diamond 3-4 stones/carat ---------------------|
Natural diamond 5-6 stones/carat →
Cation exchange capacity vs. PDC profile. (See Reference 1)This relates to the poten-
tial for bit balling in water-based muds.

CEC 40 35 30 25 20 15 10 5

Fish tail design ------------------------------------------------------|
Large cutter applications --------------------------------------------------------|
Conventional cutter sizes -------------------------------------|
(Cation exchange capacity is approximately API gamma ray units ÷ 4)

191