Page 642 - Air and Gas Drilling Manual

12-6 Air and Gas Drilling Manual
have their own single-shot system and take position readings every few hundred feet
of drilled depth (measured depth) (most governmental agencies require survey data at
500 ft intervals or less). These single readings can be plotted to show the three-
dimensional location of the borehole.
The multiple-shot instrument is operated very much like the single-shot
instrument except that it takes many photographs of the compass/pendulum. The
time intervals for the multiple camera shots can be pre-set in the instrument. Thus,
if a continuous set of position points are needed, the instrument can be lowered (on a
slick line) to the bottom location in the drill string and instrument slowly pulled out
of the well so that the camera can take photos of the compass/pendulum at pre-
determined depths. In this manner, a reliable three-dimensional location of the
surveyed borehole section can be prepared.
Magnetic survey instruments are durable downhole tools and if properly
“hardened,” can survive high vibration environments.
Gyroscopic Instruments
Conventional gyroscopic survey instruments are designed to take stationary
readings referenced to an initial directional gyroscopic alignment. This initial
directional gyroscopic alignment is set at the surface prior to running the instrument
into the well. This alignment is preserved by using mechanical gimbals.
Gyroscopic instruments are designed either to take stationary readings based on
the rate gyroscopic response to the earth’s rotation, or to take readings of the sensor
outputs as the tool moves through the borehole. These sensor outputs are referenced
to the original sensor alignment (e.g., the tool face and bent sub) to calculate tool
attitude.
There is a special class of high end gyroscopic survey instrumentation that are
available on a limited basis. These gyroscopic instruments utilize three ring-laser
gyroscopes and three accelerometers in combination with precision depth
measurement to achieve high speed, high accuracy, real-time borehole position
surveys. These newer class of gyroscopic survey instrument is the basis of most
new MWD systems.
Air and Gas Drilling Applications
The magnetic single-shot and multishot instruments are not used while drilling
is progressing. These instruments are run in the well when all drilling operations
have ceased. Thus, when used simply as survey tools, these downhole instruments
are not subjected to the high drill string vibrations that characterize compressed air
(or other gas) drilling operations. The magnetic downhole instruments are usually
used to obtain a three-dimensional plot of the borehole at drilling stoppage intervals.
Once the magnetic instruments are retrieved, the photos developed (usually at the rig
site) and the position readings entered into a computer, calculations are made and the
borehole trajectory plotted. This borehole trajectory plot can be used by a directional
driller to make the appropriate mechanical corrections to the drill string (i.e.,
orientation of the tool face of the bent sub) to improve the accuracy of a directional
drill operation. However, when using magnetic instruments such corrections cannot
be made rapidly (as in an MWD system). Using magnetic survey instruments in a
directional drilling operation is not a real-time process. But in some directional
drilling operations, particularly small diameter boreholes, the use of magnetic survey
instruments to control borehole trajectory is quite cost-effective. In these drilling
situations, even the single-shot instrument can be used to give position data that can