Page 317 - Drilling Technology in Nontechnical Language

308 Drilling Technology in Nontechnical Language Second Edition

If a small-diameter pilot hole is drilled and later opened up to a larger
hole, the losses situation can be controlled much more easily. The mud

flows faster in a small pilot hole annulus, lifting cuttings out faster. The
physical area of the wellbore wall is smaller, and thus there is less area
from which the mud can escape. When the hole is opened up, the ROP is
controlled at a rate that prevents excess loading in the annulus by cuttings.

On platform wells, if all of the conductors are set before drilling
begins, it is possible for losses to become established between the well
being drilled and an adjacent conductor. The distance through the rock
can be very short (a few meters). (See figure 4–1 to see how close together

the wells can be when drilled from a platform or template.) The other very
real danger here is that if shallow gas is penetrated by the well, gas might

channel through the rock and flow up and out via an adjacent conductor that
forms a perfect conduit to just below the platform and without any diverter
set on it. In this situation, conductor depths may be staggered to increase
the distance between conductor shoes. It is also very good practice to set
around 50 ft of cement in the bottom of each conductor before starting to

drill the first well.

Losses in normally pressured, deeper formations

These formations may be unconsolidated, naturally fractured,
or fractured by the drilling operation. The formations also may be
consolidated but highly permeable, with pore sizes too large for the mud
solids to plaster. The loss zone can be anywhere in the open hole, not
necessarily the formation just drilled into.
Several factors can contribute to the mud loss, such as annulus loaded
with cuttings, high equivalent circulating density (ECD), excessive mud
density, insufficient mud viscosity, and high water loss (low solids content

to plaster the wall and plug off high-permeability formations). Other
contributing factors could be excessive surge pressures, breaking the
formation during a formation integrity test, or closing in the well after
a kick.
It is necessary to identify the type of loss zone and the mechanism
causing the losses to start. Knowing the depth and type of loss zone will
help formulate a strategy to cure the losses.

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