STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Muds and Completion Fluids 705
The filtercake plugging of perforations or fractures is usually difficult to
remove through acidizing or backflowing. The solutions are:
* use of solids-free brine
* use of bridging solids that are acid/oil soluble
* use of commercial bridging materials (large fractures or perforations)
The compatibility of invading fluids with pay zone rocks may relate to swelling
clays, water blocking, or emulsion blocking. In many sandstone reservoirs there
are agglomerations of clay minerals and other fine formation particles are in
equilibrium with the pore fluids. If the existing brine is displaced with a lower
salinity fluid from the completion fluid, swelling clays such as montmorillonite
or some illites can expand, and non-swelling clays such as kaolinite can disperse.
The swelling and disaggregation can lead to a blocking of the pores.
In the water-blocking mechanism large volumes of invaded liquid may be
retained by low permeability or low-pressure formations. The blocking may occur
for an oil wet and a water wet sandstones.
The design factors to prevent blocking involve the use of low-viscosity fluids
with minimum interfacial tension, minimum capillary pressure, and minimal
fluid loss.
The emulsion blocking mechanism involves formation of emulsion in the pores
either by self-emulsification of water-based filtrate with the crude oil, or oil
filtrate from an oil-based fluid emulsifying formation water. The emulsions are
viscous and can block the pores. The remedial design is to prevent emulsification
either by eliminating oil from completion fluid or by the use of demulsifiers.
Components in the invading water-based filtrate and in the formation waters
may react to form insoluble precipitates which can block the pores and give rise
to skin damage. The scale can be formed by interaction of calcium-based brines
with carbon dioxide or sulfate ions in the formation water. Alternatively sulfate
ions in the invading fluid may react with calcium or barium ions in the
formation water. Analysis of the formation water can identify whether such a
problem may arise.
Table 4-62 contains a checklist for proper selection of completion/workover fluids.
CompletionMlorkover Fluid Systems
Selection of completion/workover fluid system is entirely dependent upon its
function, which, in turn, depends on the completion method. The method may
involve underreaming, gravel packing, perforation, or workover. Completion
fluids used for underreaming have to display formation bridging and low spurt
loss and filtrate loss to support the sand and prevent sloughing. Because the
filter cake will be trapped between the gravel pack and the formation, the fluid
should be composed of particles, soluble in acid or oil, and small enough not
to bridge off the gravel pack when the well is flowed.
Gravel packing completion fluids should exhibit sufficient viscosity to carry
and place the gravel efficiently. However, high gel strengths for prolonged
suspension are not necessary. Thus the polymer solution can easily flow out of
the pack on production. Also, the solution can be formulated with a breaker
(enzyme or oxidizer) such that the viscosity is completely broken allowing
complete cleanup. Normally, filtrate loss control is not employed in the gravel
carrying fluid.
Low-density perforating completion fluids for underbalanced perforation
greatly reduce the possibilities of plugging. If overbalance perforation is needed,