Page 644 - Air and Gas Drilling Manual

12-8 Air and Gas Drilling Manual
the drill pipe near the surface (secured to the drill pipe) to a side-entry sub. The
side-entry sub is in the drill string near its top. The hard wire plugs into a plug
connection in the side-entry sub. In the inside of the side-entry sub is a similar plug
connection that connects to another active wire line that runs inside the drill string to
the survey instruments in the BHA. The steering tool shown in Figure 12-2 uses
magnetic survey instruments to give compass/pendulum readings to the surface in
real-time. The survey package in the BHA is located just above the bent housing
(sub) and is referenced to the tool face of the bent housing. Since this is a magnetic
survey instrument, the survey package must be in a nonmagnetic (Monel alloy) drill
collars. With the survey package referenced to the bent housing tool face, the surface
operator always knows how the compass/pendulum readings relate to the position of
the bent sub. The steering tool example in Figure 12-2 makes use of a PDM to
rotate the drill bit. The downhole motor together with the bent housing allows the
housing tool face to be oriented. When the tool face is properly oriented, the entire
drill string can be slid along the low side of the borehole as the drill bit is advanced
in a pre-determined direction. Figure 12-2 shows an example of a side track
operation where a cement plug is used to force the drill bit to drill into the rock
formation on the side of the borehole. When it is necessary to drill straight after the
direction corrections have been made and the directional portion of the well
completed, the drill string can be raised to the side-entry sub and the hard wire
section on the outside of the drill string removed. The drill string can then be
lowered into the well and the drill string slowly rotated as the combined PDM and
drill string rotation advance the drill bit. In this manner, the effect of the bent
housing can be averaged-out and the borehole advanced along a more or less straight
directional course. Obviously, any downhole motor that can operate on compressed
air (or other gas), aerated drilling mud, or stable foam can be used with a steering
tool to provide a directional drilling capabilities for air and gas drilling operations.
Steering tools have had moderate success in providing directional drilling
capabilities for air and gas drilling operations. These successes have generally been
confined to larger diameter borehole directional operations. Steering tools are
available in tool outside diameters from 9 1/2 inches to 3 3/8 inches in diameter
(i.e., for borehole diameters from 17 1/2 inches to 4 3/4 inches).
Electromagnetic MWD
The electromagnetic MWD downhole tool transmits its downhole survey
measurements by emitting electromagnetic waves which are received at a surface
antenna, processed by a computer, and outputted as printouts and trajectory plots
that can be used by the directional driller to make drill string corrections. The
electromagnetic waves carrying the survey data transmits through the rock formations
between the downhole tool and the surface (see Figure 12-3). The electromagnetic
transmission does depend on the rock types being traversed by the waves, thus, the
operational capability of this MWD system can be depth limited. This
electromagnetic telemetry system does not depend on a particular type of drilling
fluid being in the well, thus, the system can be used with any rotary drilling fluid
system. The electromagnetic telemetry system operates on a long-life battery
subsystem, therefore, no drilling fluid driven on-board generator is needed.