Page 189 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 165

2.3.2
Directional Planning [ ]

Since the Totco tool only gives an inclination reading, the wellbore
position cannot be calculated. Also, it does not detect a low-angle spi-
ral hole where inclination stays about the same but azimuth constant-
ly changes. Therefore, the tool is only used in vertical wells to ensure
that the well stays within a certain limit of verticality. A gyro or multi-
shot survey will have to be run later if a definitive wellbore path and
bottom hole location are needed.
Magnetic single shot (MSS) surveys. A MSS tool consists of an
outer barrel that houses the working parts of the tool. The measure-
ment part consists of a camera unit with timer mechanism, combined
compass, and inclinometer.
There are three different camera units that use either a timer set at
surface, a motion sensor, or a Monel detector that fires one minute after
it detects the presence of a Monel drill collar. If the survey will be run
on wireline, the motion sensor or Monel detector is preferable if the
hole is not sticky, since it will fire one minute after reaching bottom. If
the survey barrel is to be dropped, either to be fished on wireline or left
in the string during a trip out, then use the timer unit.
The camera takes a picture of the compass/inclinometer unit onto
a disk of film which is developed on surface. Inclinometer units come
in different angle unit ranges, generally 0˚-10˚, 0˚-30˚, and 0˚-90˚.
Choose the next range up from the maximum anticipated inclination.
The MSS can also give toolface azimuth if it is dropped into an ori-
entated holder, such as a UBHO sub. On the film, the TFA appears as
a line on the compass rose.
As the MSS gives inclination and azimuth, the wellpath and bottom
hole location can be calculated as long as you know the location of the
previous casing shoe. Generally, a gyro or magnetic multishot survey
will have been run at the previous casing point to give a definitive shoe
position. Some wireline logging tools also give deviation surveys and
most, if not all, modern MWD tools give surveys of sufficient accuracy
to be definitive.
For running the MSS to give azimuth, nonmagnetic or monel drill
collars need to be run over the depth of the survey tool. Refer to “Monel
drill collars and stabilizers—selection and use” later in this section.
Magnetic or gyro multishot surveys. At the end of a hole section,
if monel drill collars have been run in the BHA, a magnetic multishot
tool can be dropped. This produces a strip of film with shots taken at
regular timing intervals. While tripping out, the surveyor takes accu-

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