Lost circulation is among the most common
drilling problems. It can be started at shallow, unconsolidated formation and
extend to the deep consolidated formation which can be fractured by
inappropriate well design or drilling operations.
The drilling fluids can flow freely into the
shallow formations due the high level of permeability of this surface
formations. Drilling can be continued in such circumstances but it can be risky
for the operations, following this procedure can cause underground cavities.
This situation can be corrected by multiple solutions like using lost
circulation material, reducing flow rate, controlling rate of penetration, thickening
the drilling mud by using lime or cement.
High level of rate of penetration can increase
the effective mud density leading to mud losses. The fast drilling has to be
accompanied with high flow rate to clean the well bore, these two factors have
to be adjusted together for safe operations. If the mud density out of the well
bore is significantly higher than the mud weight used in the active system,
then many actions can be applied like optimizing mechanical treatment of mud,
reducing rate of penetration or at the limit case the operations can be ceased
to allow for better hole cleaning.
There are in some regions where the drilling
can be performed without flow return. The generated cuttings can be lifted from
the bottom, carried and deposited at the lost-circulation zone. The parameters
must be watched closely like torque and drag.
Seepage losses can be faced when drilling hard rocks with naturally fractured formation. In such situation using lost circulation material can be enough to overcome these losses until drilling this zone.
Fractures can be induced by the imposed hydrostatic pressure, flow dynamic forces and pipe movement. This can happen in normal
pressured formations above a pressure transition zone. The transition zone is
that section where the pore pressure deviates from the normal trend. Loss circulation at these zones can allow the fluids to flow from the
abnormal pressured formation into the well bore leading to complicated problems
(well kick and lost circulation).
As a precaution for this problem, the cement
around the last casing shoe should be tested, this can be done by performing shoe
bond test. In some cases, the formation tests can be performed many times at
many points of the well to adjust the mud weights or to get more information
before performing the next cement job. Missing data before cementing can lead
to severe losses and non-protected casing due to the high density of cement,
high friction losses which can increase the effective bottom density.
In the case of the severe losses or total loss
of circulation and with confirmation that no inflow from the formation into the
well bore existed, the drill string can be pull out of the hole and open ended
pipe can be run to place a cement plug. This is not a general discussion, it is
a crucial field decision that can be followed by analyzing the situation and gathering
maximum of information.
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