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Chapter 13 – DRILLING PROBLEMS AND SOLUTIONS 317
second possibility is that the hydrostatic pressure exerted by the mud is
insufficient to hold the formation together, and slivers of shale fall off into
the well as cavings (mentioned in chapter 3). This also can occur when
drilling in overpressured shales. The pore pressure might even be higher
than the mud hydrostatic pressure, but the well does not kick due to the
very low permeability of shale, and high levels of cavings result. Even if
the annulus is clean of cuttings at the start of a trip out of the hole, reactive
formations can cause material to fall in during the trip and stick the pipe.
The warning signs are that the force needed to pull the drillstring out does
not decrease as rapidly as would be expected as steel is removed from the
hole. If this occurs, the driller must stop pulling out and circulate the hole
clean again.
In shallow, unconsolidated sands, tophole collapse might occur. The
hydrostatic pressure must be able to support the formation, but if there is
not a good filter cake and losses occur, there is little difference in pressure
between the formation around the wellbore and mud hydrostatic pressure.
The sand simply falls into the well. (This problem can be demonstrated by
trying to dig a deep hole on a beach.) The mud must have good plastering
characteristics so that a firm filter cake is formed, allowing the mud
hydrostatic to hold the sand back.
Sometimes, formations are fractured. This can happen especially with
brittle shales, coal, and limestone. Mud gets into the fractures, and this
lubricates the fracture faces as well as changing the pressure regime near
the wellbore. Pieces of formation fall off (fracture cavings). Cavings from
fractured formations can be identified by their size, shape, and presence of
fracture faces that may be visible.
Junk in the hole can also fall into this category. If something is lost
down the hole, either because of something breaking or by something
falling through the rotary table, stuck pipe may result.
Cement that is not fully set is known as green cement. If a cement plug
was set in the hole and the driller runs into it with the bit while it is still
green, the pipe can become plugged or can get stuck in the cement.
Differentially stuck pipe
Differentially stuck pipe is far less common today than 15 or 20
years ago (fig. 13–5). The knowledge of the mechanisms involved, better
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