Page 217 - Well Control for Completions and Interventions

210 Well Control for Completions and Interventions

5.9.3 Bridging solids
In high permeability or fractured formations, it may not be possible to
control losses using viscosity alone, and it may be necessary to mix bridg-
ing solids into the viscous pill. The most commonly used bridging fluids
are, in order of popularity, calcium carbonate (CaCO 3 ), sized common
salt (NaCl), cellulose fibers, and oil soluble resin.
Bridging solids work most effectively when they are approximately
one-third to one-fifth of the formation pore space diameter. Pore diame-
ter can be measured from core, or if pore diameter is unknown it can be
estimated. An often used short cut is to take the square root of permeabil-
ity to represent the average pore diameter in microns (from the Kozeny
equation). For example, a formation with 1 darcy (1000 md) permeability
would have an average pore diameter of approximately 32 µm. Bridging
particles would therefore need to be between 6 and 11 µm. In reality,
bridging material for a 1000 md formation would have a wider range of
particle size, for example, 1 µmup to 30 or 40 µm. A wide distribution
provides particles large enough to bridge across a range of pore sizes (the
calculation only provides an average), while containing enough smaller
particles to create a low permeability cake.
Properly sized bridging solids should spontaneously lift off when the
well is put into production. However, since the particles cover a wide
range of sizes, it is possible than some particles will end up plugging the
pores and will not be removed by flow.

5.9.3.1 Calcium carbonate
Calcium carbonate (CaCO 3 ) is the most common of all the bridging
materials. Usually made from dolomite or marble, it is available in a wide
range of sizes or can be made to a particular specification. Any residual
damage caused by particulates not lifting off when the well is flowed can
be removed using hydrochloric acid (Fig. 5.7).

5.9.3.2 Sized salt
Sodium chloride (NaCl) particles are frequently used as a bridging mate-
rial. To prevent the solid salt particles from dissolving, the carrier fluid
must be saturated with respect to sodium chloride. In theory, solution of
the salt by formation brine or a low salinity wash should effectively
remove the filter cake. However, in practice, clean-up effectiveness is
reduced by the polymers used to keep the salt particles in suspension.