Peh: Drilling Problems and Solutions Publication Information Petroleum Engineering Handbook

Fig. 10.5—Lost-circulation zones.
Loss of Circulation
Definition
Lost-Circulation Zones and Causes

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Formations that are inherently fractured, cavernous, or have high permeability are potential
zones of lost circulation. In addition, under certain improper drilling conditions, induced
fractures can become potential zones of lost circulation. The major causes of induced fractures
are excessive downhole pressures and setting intermediate casing, especially in the transition
zone, too high. 
Induced or inherent fractures may be horizontal at shallow depth or vertical at depths greater
than approximately 2,500 ft. Excessive wellbore pressures are caused by high flow rates (high
annular-friction pressure loss) or tripping in too fast (high surge pressure), which can lead to
mud ECD. In addition, improper annular hole cleaning, excessive mud weight, or shutting in a
well in high-pressure shallow gas can induce fractures, which can cause lost circulation. Eqs.
10.6 and 10.7 show the conditions that must be maintained to avoid fracturing the formation
during drilling and tripping in, respectively. 
....................(10.6) 
....................(10.7) 
where λ
mh
= static mud weight, Δλ
af
= additional mud weight caused by friction pressure loss in
annulus, Δλ
s
= additional mud caused by surge pressure, λ
frac
= formation-pressure fracture
gradient in equivalent mud weight, and λ
eq
= equivalent circulating density of mud. 
Cavernous formations are often limestones with large caverns. This type of lost circulation is
quick, total, and the most difficult to seal. High-permeability formations that are potential lost-
circulation zones are those of shallow sand with permeability in excess of 10 darcies. Generally,
deep sand has low permeability and presents no loss-of-circulation problems. In noncavernous
thief zones, mud level in mud tanks decreases gradually and, if drilling continues, total loss of
circulation may occur.
The complete prevention of lost circulation is impossible because some formations, such as
inherently fractured, cavernous, or high-permeability zones, are not avoidable if the target zone
is to be reached. However, limiting circulation loss is possible if certain precautions are taken,
especially those related to induced fractures. These precautions include maintaining proper mud
weight, minimizing annular-friction pressure losses during drilling and tripping in, adequate
hole cleaning, avoiding restrictions in the annular space, setting casing to protect upper weaker
formations within a transition zone, and updating formation pore pressure and fracture
gradients for better accuracy with log and drilling data. If lost-circulation zones are anticipated,
preventive measures should be taken by treating the mud with lost-circulation materials (LCMs).
When lost circulation occurs, sealing the zone is necessary unless the geological conditions allow
blind drilling, which is unlikely in most cases. The common LCMs that generally are mixed with
the mud to seal loss zones may be grouped as fibrous, flaked, granular, and a combination of
fibrous, flaked, and granular materials. 
These materials are available in course, medium, and fine grades for an attempt to seal low-to-