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).
Since the magnetic single-shot instrument utilizes a simple compass for directional information, the instrument must be placed in a nonmagnetic portion of the drill string in order for the compass to give accurate azimuth readings. This is why the nonmagnetic drill collars are placed at the bottom of the drill string. When running the single-shot survey care must be taken to make sure that the single-shot instrument is located approximately midway along the nonmagnetic drill collar length section before the camera film is allowed to be exposed.
More details regarding directional drilling operations and surveys will be discussed in Chapter 12.
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