Geology Reference
In-Depth Information
7.1.1 Stable-closed designs.
The model in Figure 7.1, which is “obvious” in concept, causes significant
operational problems. Rock particles and other debris present in the drilling
fluid tend to lodge between the axial clearance or gap between the rotor and the
stator, thus impeding rotary motion. Usable gap distances are almost always
small, say much less than 1/8 inch, since larger spacings do not create detectable
surface signals. This “1/8 inch rule” is known from simple valve tests, e.g.,
larger gaps produce very small water hammer pressures (strong areal blockage is
required to produce the plane waves that ultimately reach the surface). When
rotary motion is stopped, the rotor surprisingly stops in the closed position: the
solid lobes of the upstream rotor completely block the opened ports of the
downstream stator, with this closed position completely stable and resistant to
any attempts to re-open the valve.
This stable-closed behavior has several undesirable operational
consequences. (1) High pressures developed in the drillpipe induce the
mudpump seals to break, causing possible pump damage and introducing a rig
floor safety hazard. (2) Excessive pressures may fracture or damage the
formation, and most definitely, increase unwanted invasion. (3) High pressures
will damage the MWD tool, while high flow rates through the narrow gap will
severely erode the rotor and stator. (4) The need to remove the tool, especially
from deep wells, means a loss of expensive offshore rig time, not to mention
additional formation invasion while tool retrieval is in progress.
7.1.2 Previous solutions.
In the 1970s, several methods were developed to solve this problem. The
jamming problem was first discussed in detail in U.S. Patent No. 3,770,006.
The inventors noted that “. . . in logging-while-drilling tools of this type, the
signal generating valve normally develops certain hydraulic torque
characteristics as a function of the flow rate through the valve which tend to
force the valve to its closed position. This creates problems as drilling mud is
pumped down the drill string and through the valve before the tool begins
operation and the motor begins to power the valve.” This is crucial when power
is produced by a mud turbine downstream of the valve, of course, and less
crucial for battery powered tools.
A simple mechanical solution is given. Essentially, “a spring means is
included in the tool which has sufficient force to bias the rotor upwardly away
from the stator when a low rate of flow is passing down the drill string.”
Recognizing that the “gap in the tool must be relatively small during operation
of the tool in order for the signal generated by the valve to have sufficient
strength to reach the surface,” the authors proceed to note how as “the flow rate
increases, the pressure drop across the rotor also increases. When the pressure
drop exceeds the force of the spring means, the rotor moves downward toward
the stator which establishes the gap necessary for satisfactory operation of the
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