Page 643 - Air and Gas Drilling Manual

Chapter 12: Directional Drilling Operations 12-7
be plotted with a computer to give a good quality borehole trajectory. This
trajectory can then be used to make tool face corrections to improve subsurface target
intercept accuracy.
The basic downhole gyroscopic survey instrument can be used in much the same
manner as the magnetic survey instruments discussed above. These instruments can
be run into the well when drilling has ceased and borehole attitude versus depth
readings taken as the instrument is withdrawn from the well. These simple
gyroscopic survey instruments are hard wired to the surface with an active wire line.
Thus, the data coming from the well can be used in real-time to update a borehole
trajectory plot. Once the trajectory is known, appropriate mechanical corrections can
be made to the drill string by the directional driller to improve the accuracy of the
directional drilling operation. The main advantage of the gyroscopic survey
instruments is that they can be used in any part of the well (cased and openhole) and
do not require nonmagnetic drill collars in the drill string.
12.2.2 MWD Equipment
MWD downhole and surface equipment utilize survey data in a real-time
analysis process to give an updated trajectory of a directional borehole and, with
appropriate software, a prediction of the future borehole path. This knowledge
allows the directional driller to make appropriate mechanical corrections in drill
string orientation (called “steering” the drill string) that will allow the advancing
drill bit to hit an intended subsurface target area. An MWD requires that the
downhole survey instrument operate in the drill string as drilling progresses. Both
the magnetic and gyroscopic survey instruments can be used as the basic survey
tools for MWD systems. Once the survey data has been obtained (either by
magnetic or gyroscopic), the MWD system must provide a way to get this
information to the surface where the directional driller can act on the information and
make appropriate drill string orientation adjustments.
Conventional drilling mud operations have MWD systems that make use of an
acoustic mud pulse signal communication systems that provides nearly real-time
survey information to the surface operators (via a pulse generated binary code).
However, this mud pulse system does not work in compressed air (or other gas)
drilling fluids, in aerated mud drilling fluids, or in stable foam drilling fluids. The
compressed gas dampens and scatters any acoustic signals generated at the bottom of
the drill string by these pulse systems.
Also, particularly in compressed air (or other gas) drilling, the rather delicate
gyroscopic survey instruments on-board most MWD systems are easily damaged in
the high drill string vibration environment characterized by air and gas drilling
operations. Some attempts to “harden” these instruments to survive these forces
have been successful, particularly for the larger diameter downhole instruments. But
this remains a problem for smaller diameter downhole instruments. The magnetic
survey instruments are generally harder and, thus, more survivable in high drill
string vibration environments.
Steering Tools
The steering tool is an early type of MWD system. Steering tools have a hard
wire connection which runs from the downhole survey instrument package in the
BHA to surface computer and output printer and plotter. Figure 12-2 shows a
schematic of this type of system. The active hard wire is run down the outside of