Page 325 - Drilling Technology in Nontechnical Language

316 Drilling Technology in Nontechnical Language Second Edition

Fig. 13–4. Hydrated clays sticking to a drill collar

Stable cuttings beds can form in inclinations over about 60°, especially
if the hole is overgauge in places. At lower inclinations, any cuttings
bed will get only so large and then will avalanche down the hole. Stable
cuttings beds sit there nice and quietly until the BHA is tripped out of the
hole. As the larger BHA encounters the bed, it ploughs into the pile of
solids and can get stuck. If the pipe is packed off with solids, the prognosis
is bad. To cure the problem, circulation must be reestablished, but this is
often impossible.

Another source of solids in the well is reactive formations. This is
overwhelmingly a problem with shales. Formations can become unstable
due to adverse reactions with the mud physical or chemical properties.

Two instability modes are possible. The first possibility is that the shale
hydrates, becomes plastic and sticky, and falls into the wellbore. The

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