Page 198 - Drilling Technology in Nontechnical Language

Chapter 8 – DIRECTIONAL AND HORIZONTAL DRILLING 189

succession of these points then feeds into calculations to work out the path
of the well, as described below.
There are a number of tools that may be used to produce one or more
survey points.
1. Magnetic single shot survey. Historically, a slim, stainless-steel
barrel of around 1½" diameter contained a magnetic compass
unit, an inclinometer, and a camera controlled by a timer. A small
film shaped like a disc was loaded into the unit, and the survey

barrel was lowered into the well. It could be run on wire, or
dropped down the drillstring and fished with wireline, or dropped

down the drillstring before pulling out of the hole. The timer
would then be set to give enough time for the tool to be run to the
right depth. The camera would take a photograph of the compass,
which also had a marker on it showing the inclination. Modern
units still make the measurements but dispense with the camera
and film. A less common unit has a motion sensor that takes the

survey two minutes after no motion is sensed.
2. Magnetic multishot survey. This tool records a series of
surveys as it is pulled out of the hole. Originally the surveys

were recorded on to a strip of film, which had to be developed
and manually viewed; now the surveys can be recorded in tool
memory or data may be transmitted real-time to the surface.
3. Gyro multishot survey. Normally run on wireline that transmits
information back to the surface, the GMS can take many readings
at short distance intervals. The surface computer knows the depth
of each survey. The more surveys are taken, the more accurately
the well path can be calculated. Gyroscopic tools are an order of
magnitude more accurate at reading azimuth than magnetic tools.
In addition, there are factors that affect magnetic tools, such
as electrical storms and local magnetic anomalies, that can be
impossible to completely account for.
4. Measurement while drilling. A barrel the size of a drill
collar (around 8" outside diameter) contains instruments that
measure direction with magnetic tools and inclination with an
inclinometer. This information is transmitted to the surface. The
depth of the survey is known, as its position within the drillstring
is known.

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