Geology Reference
In-Depth Information
Just beneath the bit, a low pressure zone is created, because the drillbit
pulls away from the drilling mud. This under-pressure propagates downward to
the bottom of the rigid hole, where it must reflect with the same negative sign
according to Example 1-1. This low pressure wave travels up the annulus
through the small bit bypass area, where it adds to up-going high pressure wave
created after the initial instant. Thus, there is some neutralization of the upgoing
signal, at least after the initial instant. The physics at the bottom of the hole is
much more interesting. From Chapter 1, we found that the magnitude of any
pressure wave doubles at a rigid termination in a one-dimensional waveguide.
Thus, the under-pressure traveling toward the hole bottom from the bit is twice
as low there as otherwise. This underbalance means that influx of higher
pressure formation fluids is possible, and thus increases the probability of
operational hazards. In general, pulling and running pipes in boreholes induce
pressure surges that can cause lost circulation, fractures and formation fluid
influx, resulting in gas kicks and blowouts. Accurate prediction methods are
required, especially in ultra-deepwater wells where pressures must be
maintained within narrow limits. The stable and robust difference models given
in this topic permit the development of field operable software algorithms
requiring minimal computing resources.
A
1
Pipe
A
2
Collar
Reservoir
Drillbit
A
3
Hole Bottom
Figure 5.3. Drillstring in borehole.
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