Oi Marisa,

Pack off tem na verdade vários sentidos. Nesse caso entendo que seja o de "obstrução". Veja o que diz abaixo o dic. da Schlumberger.

Espero que ajude.

When the well packs off, there is a sudden reduction or loss of the ability to circulate, and high pump pressures follow. If prompt remedial action is not successful, an expensive episode of stuck pipe can result. The term is also used in gravel packing to describe the act of placing all the sand or gravel in the annulus.