In the early 1980’s, the directional drilling industry relied on nonmagnetic drill collars as an
essential tool. These tools frequently failed due to stress corrosion cracking that started on the bore
surface. A partial solution was developing mechanical treatments that produce a thick compressive
residual stress layer on the bore surface. Since then, compressive stress layers have proven to be
an effective means to prevent bore cracking and thick compressive layers have proven 100 %
effective. Furthermore, compressive residual stress treatments are now standard on the bore of
nonmagnetic drill collars and, consequently, bore cracking has almost disappeared. Bore treatments
for nonmagnetic drill collars will be reviewed, including some details of current bore treatment
specifications. A pending American Petroleum Institute specification for bore treatment of
nonmagnetic drill collars will be mentioned.
Bore treatment technology has had a greater impact than simply preventing the stress corrosion
cracking problems that existed 25 years ago. Since commercial introduction of bore treatments in
1983, a lot more nonmagnetic directional drilling equipment now exists for two reasons.
a. In the decade of the1980’s, horizontal drilling technology was developed and, after 20 years
commercial progress, there were 600 rigs drilling horizontal wells in 2008 in the United States.
b. From a beginning in 1978, Measurement While Drilling (MWD) technology has been growing.
Many nonmagnetic tools with a bore treatment are currently used within MWD systems.
Keywords: nonmagnetic drill collar, non-magnetic, NMDC, directional drilling, horizontal drilling,
measurement while drilling, MWD, residual stress, compressive stress, stress corrosion cracking