Page 91 - Hydrocarbon Exploration and Production Second Edition

78 Drilling Problems

4.7.3. Lost circulation
During drilling operations large volumes of drilling mud are sometimes lost into a
formation. In this case, normal mud circulation is no longer possible and the fluid
level inside the borehole will drop, creating a potentially dangerous situation as
described below. The formations in which lost circulation can be a problem are
a highly porous, coarse or vuggy formation which does not allow the build-up of an
effective mudcake
a karst structure, that is a limestone formation which has been eroded resulting in
a large-scale, open system comparable to a cave
a densely fractured interval
a low-strength formation in which open fractures are initiated by too high a mud
pressure in the borehole.
The consequences of lost circulation are dependent on the severity of the losses,
that is how quickly mud is lost and whether the formation pressures in the openhole
section are hydrostatic or above hydrostatic, that is overpressured (see below). Mud
is expensive and losses are undesirable but they can also lead to a potentially
hazardous situation. Moderate losses may be controlled by adding ‘lost circulation
material’ (LCM) to the mud system, such as mica flakes or coconut chippings. The
LCM will plug the porous interval by forming a sealing layer around the borehole
preventing further mud invasion. However, LCM may also plug elements of the
mud circulation system, for example bit nozzles and shale shaker screens, and may
later impair productivity or injectivity of the objective intervals. In severe cases, the
losses can be controlled by squeezing cement slurry into the trouble horizon. This is
obviously not a solution if the formation is the reservoir section!
If sudden total losses occur in a hydrostatically pressured interval, for example in
a karstified limestone, the decision may actually be taken to drill ahead without
drilling mud but using large quantities of surface water to cool the bit. The fluid
level in the annulus will usually stabilise at a certain depth; this type of operation is
also referred to as ‘drilling blind with a floating mud cap’. Since no cuttings are
returned to surface, mudlogging is no longer possible, therefore preventing early
reservoir evaluation.
In the event of a sudden loss of mud in an interval containing overpressures, the
mud column in the annulus will drop, thereby reducing the hydrostatic head acting
on the formation to the point where formation pressure exceeds mud pressure.
Formation fluids (oil, gas or water) can now enter the borehole and travel upwards.
In this process, the gas will expand considerably as it loses its initial pressure due to
the reduction of hydrostatic head above the gas bubble. The last line of defence left
is the BOP. However, although the BOP will prevent fluid or gas escape to the
surface, ‘closing in the well’ may lead to two potentially disastrous situations:
1. Formation breakdown (fracture development) in a shallower, weaker formation
and subsequent uncontrolled flow from the deeper to the shallower formation
(internal blowout).
2. Formation breakdown and subsequent liquefaction of the near-surface strata and
the initiation of cratering below the rig. This will result in a surface blowout.