THD WELL LOGS
TECHNICAL HOLE DEVIATION WELL LOGS
Technical Hole Deviation (THD) Well Logs
graph geometric directional well plan
and actual wellbore properties. They enable
observers to monitor directional control
performance and help directional drillers to rationalize
directional control actions performed between survey
stations.
THD component values are plotted versus actual measured depth
(MD). The Vertical THD Log addresses
deviations in the "vertical" sense. It presents planned and actual
values of wellbore inclination and dogleg-severity (DLS), and THD components VD, RCVD,
ID, and RCID.
Similarly, in the "horizontal" sense the Horizontal THD Log presents planned and actual
values of wellbore azimuth and dogleg-severity, and THD components
HD, RCHD,
AD, and RCAD.
Regardless of the planned
well path linear/curved 3D/2D complexity, the target path is always
a single vertical green zero line on the THD
track.
Like standard directional
plots, THD logs can be created prior
to drilling and present the plan.
Then, as drilling commences, the "actual parts" of THD are
plotted and directional control performance unfolds. The footer section of a
THD log contains standard vertical section and plan view
directional plots.
THD WELL LOG HEADERS
Planned and actual DLS are
graphed on the inner left track of both the Vertical
and the Horizontal THD Log. Depths with directional
survey stations are marked with unfilled circles. Circular filled markers
on the planned DLS curve convey critical-point well
plan changes (e.g.,
curved to straight hole section).
Vertical THD Log Header
The log header
section contains the following analytics:
Excess Measured Depth
- amount of actual hole drilled subtracted by the corresponding planned
measured length of drill hole. This value may indicate the
"cost" of oscillating about the planned path. However, in some
cases this value can be negative such as when
dropping a wellbore to vertical with actual DLS exceeding
planned DLS.
Average Absolute VD
- average absolute value of vertical deviation (VD) associated with real
survey stations over the length of drill hole for which the THD log was
created. A smaller value usually indicates better directional control
performance.
Average Absolute ID
- average absolute value of inclinational deviation (ID) associated with
real survey stations over the length of drill hole for which the THD log was
created. A smaller value usually indicates better directional control
performance.
THD Grid Values -
convey the grid values of the THD tracks as a result of auto-scaling. For
the example header below, "20~2" means the grid value for the
VD track is 20 feet and the grid value for the ID track is 2 degrees.

The outer left track graphs planned and actual wellbore
inclination. Technical hole deviation in the "vertical" sense is displayed on
the two right tracks. The inner right track displays vertical deviation (VD-blue)
and the relative change in vertical deviation (RCVD-red).
The outer right track displays inclinational deviation (ID-blue)
and the relative change in inclinational deviation (RCID-red). A zero line (green) centers each of the two
THD tracks. The actual wellbore was drilled "on depth" and
"on inclination" only when VD, RCVD, ID, and RCID all trace their respective zero
lines.
Horizontal THD Log Header
The Horizontal THD Log header
section contains the same Excess Measured Depth values
as detailed above and the following analytics:
Average Absolute HD
- average absolute value of horizontal deviation (HD) associated with real
survey stations over the length of drill hole for which the THD log was
created. A smaller value usually indicates better directional control
performance.
Average Absolute AD
- average absolute value of azimuthal deviation (AD) associated with real
survey stations over the length of drill hole for which the THD log was
created. A smaller value usually indicates better directional control
performance.

The outer left track graphs planned and actual wellbore
azimuth. Technical hole deviation in the "horizontal sense" is displayed on the
two right tracks. The inner right track displays horizontal deviation (HD-blue)
and the relative change in horizontal deviation (RCHD-red).
The outer right track displays azimuthal deviation (AD-blue)
and the relative change in azimuthal deviation (RCAD-red). A zero line (green) centers each of the two
THD tracks. The actual wellbore was drilled "on depth"
and "on azimuth" only when HD, RCHD, AD, and RCAD all trace their respective zero
lines.
PROJECTIONS
Forecasts of VD and HD profiles are shown as thick black lines that extend from the last
directional survey
station. THD projections are helpful for directional drillers while
they rationalize directional tool setting adjustments during drilling
operations.
EXAMPLE THD WELL LOGS
THD well logs for three wells are presented below. These logs were created with
SES
and then converted to JPEG graphics files. Click a thumbnail to enlarge. All
data are from real wells.
3D Sidetrack Horizontal |
3D Sidetrack Horizontal |
2D Sidetrack Directional |
3 in. / 1000 ft |
5 cm / 300 m |
1.5 in. / 1000 ft |
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