Page 94 - Air and Gas Drilling Manual

3-22 Air and Gas Drilling Manual
3.3.3 Downhole Surveying Equipment
BHAs for drill strings used to drill deep vertical wells are usually fitted with
several nonmagnetic drill collars. These drill collars are usually near the bottom of
the BHA (just above the drill bit). These non-magnetic collars are needed to carry
out the downhole surveys required by most natural resource regulatory agencies.
Downhole surveys are used to describe the deviation of a drilled borehole from the
ideal vertical centerline of the intended well. Knowledge of where an actual well has
been drilled is needed to ensure economic and environmentally safe recovery of
mineral resources. Also such directional knowledge is used in geotechnical drilling
operations where the accuracy of the placement of a well is critical for follow-on
construction operations.
The survey is usually accomplished by using a magnetic single-shot instrument.
This instrument is usually part of the equipment inventory of a typical double and
triple rotary drilling rig. The magnetic single-shot instrument survey is carried out
by the drilling rig crew. The single-shot instrument contains a small compass which
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 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 concentric
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 wireline (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 3-20 shows a typical single-shot exposure.
This exposed single-shot picture shows a direction of magnetic north (or an azimuth
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.

Figure 3-20: Typical single-shot exposure that reads north and 1.8˚ inclination from
vertical (courtesy of Sperry-Sun Drilling Services, a Halliburton Company).