Page 215 - Well Control for Completions and Interventions

208 Well Control for Completions and Interventions

In addition to the clear and obvious need to maintain a well control
overbalance, losses need to be controlled to prevent formation damage.
Paradoxically, continual (but controllable) fluid loss can actually be an
indication that the formation is not being damaged. If fluid losses were
damaging the formation, losses would diminish, perhaps even stop. In
addition to the well control and formation damage concerns, losses can
be costly. Brine cost can be significant, some of the heavier formate
brines cost in excess of $2000 bbl.
Reducing hydrostatic overbalance would reduce losses. However, this is
unlikely to be a safe or practical option, since most completion and work-
over operations are conducted with a relatively low (200 300 psi) overbal-
ance, and any reduction would risk allowing the well to kick. Fluid loss
during completion and intervention operations is therefore almost always
controlled by placing lost circulation material (LCM) in the form of a fluid
loss control pill (FLCP) or “kill pill” across the zone of permeability. FLCPs
are made using viscous fluid, bridging solids, or a combination of both.

5.9.1 Solids-free lost circulation material

Brine viscosity can be increased by mixing HEC (hydroxyethyl-cellulose)
or XC (Xanthan gum) polymers. Concentration is normally in the range
3
of 2 4 lb/bbl (5.5 11.5 kg/m ).
Viscous fluid lost to the formation can cause formation damage, espe-
cially if they are slow to degrade. Viscous pills degrade fairly quickly if
the temperature is above 220 F. While thermal degradation is an advan-

tage, inasmuch as polymer related formation damage will be minimized,
there is the obvious disadvantage of the resumption of fluid loss. In some
cases FLCPs will need to be replenished. In low temperature wells, it may
be necessary to use breakers to remove FLCPs. Hydrochloric acid is fast
acting, but has the potential to cause formation damage. Enzyme breakers
react more slowly, but are generally less damaging.

5.9.2 Example fluid loss calculations

It is possible to estimate the effectiveness of using viscous fluids to reduce
fluid losses using Darcy’s radial inflow equation:

7:08 3 10 23 kh P r 2 P wf Þ
ð
Q 5
µB ln r e =r w 2 0:75 1 S