Page 313 - Drilling Technology in Nontechnical Language

304 Drilling Technology in Nontechnical Language Second Edition

Downhole mud losses of more than 60 bbl/hr are categorized as
serious losses. This level of losses is unlikely to be caused by high-
permeability formations. Potential causes include nonsealing faults or
fracture systems.
If the mud losses are so severe that no returns at all are seen at the
surface, the term total losses would be used to describe the situation.
Potential causes include nonsealing faults or fracture systems, and drilling
into formations that contain large caverns (also called vugs).

Losses occur because both of the following conditions are present:
▪ The drilling fluid hydrostatic pressure is higher than the problem

formation pore pressure.

▪ There is a path that allows the mud to flow into the formation and
away from the wellbore.
Preventing and curing losses addresses one or both of these two factors;
the goal is to reduce overbalance and plug off the pathways.

In discussing lost circulation, it is convenient to categorize the problem
according to situations in which the losses occur.

Losses while drilling surface hole

Losses in surface hole have two common causes:
▪ Very permeable formations (often unconsolidated sands) that
allow whole mud to seep through the pore spaces (fig. 13–1).

Commonly, losses ranging from seepage to moderate might
be caused by this. Total losses into permeable formations
are unlikely.
▪ Fractures (either existing or more likely created by the drilling
process) that allow mud to leave the wellbore. Severe or total
losses are likely with fractures.
Losses into a permeable formation might be cured by pumping lost
circulation material (LCM) mixed into some mud into the well. LCM is
bulky, lightweight plugging material that blocks the pores in the formation

taking the fluid. Materials used for LCM include clay, sawdust, mica (a

stable, nonhydrateable clay mineral with large, flat, platelike crystals) and

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