Page 91 - Air and gas Drilling Field Guide 3rd Edition

82 CHAPTER 4 Downhole Equipment

triple rotary drilling rig. The single-shot survey is usually carried out by the rig
crew. The single-shot instrument contains a small compass that floats in a liquid
and gives borehole compass direction information. The floating compass is also
designed with a half sphere top and an extended pendulum bottom. The spherical
top of the compass is etched with a traditional compass rose allowing direction
determination when viewing the compass from above and down the axis of the
instrument. Also etched on the spherical top are concentric circles that represent
different angles of inclination from the vertical. When viewing the compass from
above and down the axis of the instrument, a set of crosshairs shows the concen-
tric circles of angles of inclination. A small single-shot camera is installed in the
instrument above the compass. The camera shutter mechanism, exposure light,
and timer are battery operated. The instrument timer is set at the surface to give
sufficient time for the instrument to be lowered to the bottom of the inside of
the drill string. The instrument is lowered on a slick wire line (a simple wire line
not having electrical transmission capability). When the instrument is in place at
the bottom of the inside of the drill string, the timer actuates the light exposing
the small circular film cartridge. Figure 4-20 shows a typical single-shot exposure.
This exposed single-shot picture shows a direction of magnetic north (or an azi-

muth of 0 ) and an inclination of 1.8 from vertical. As a well is drilled, single-shot
survey pictures can be taken every few hundred feet. Calculations can be made
using these survey pictures and the measured distance to each survey point to give
a three-dimensional plot of the drilling course of the well.
Since the magnetic single-shot instrument utilizes a simple compass for direc-
tional information, the instrument must be placed in a nonmagnetic portion of

FIGURE 4-20. Typical single-shot exposure that reads north and 1.8 inclination from vertical

(courtesy of Sperry-Sun Drilling Services, a Halliburton Company).