Page 209 - Well Control for Completions and Interventions

202 Well Control for Completions and Interventions

5.6.3 Brine compatibility with the formation
Although density and crystallization temperature are the main concerns
when preparing a brine, it is also important to select a fluid system that
keeps formation damage to a minimum. During drilling operations, fluid
loss into the formation is controlled by the development of a thin, low per-
meability, filter cake on the wall of the wellbore. The solid material used to
create the filter cake (and to add density to the mud) has the potential to
damage the formation by bridging off in the formation pore spaces.
Completion and workover brines are solids-free, meaning the usual pro-
blems associated with solids blocking pore throats are eliminated. However,
the potential for damage remains. Indeed, in some poorly designed mud sys-
tems it might not be the solids content of the mud that causes the most
damage, but the loss of base fluid (filtrate) into the formation.
Damage to the formation has serious economic consequences, and
can have an indirect and negative influence on well control. The main
fluid related formation damage mechanisms are:
• Clay swelling and fines mobilization.
Many producing formations contain water sensitive clay such as
smectite (montmorillonite). These swell when exposed to freshwater,
low salinity, and (or) high pH brine. When clay expands it occupies
more of the pore space in the formation. Permeability is therefore
reduced. Freshwater or low salinity filtrate will almost always damage
formation containing swelling clays. Damage is least likely to occur if
the salinity of the brine is similar to the salinity of the formation water.
Changes in salinity or the use of deflocculating chemicals can also
lead to the mobilization of clay particles, allowing them to move
through the formation until they become trapped in pores, again
resulting in permeability reduction. Fines migration can also be caused
by high velocity flow (production or injection) (Fig. 5.3).

Figure 5.3 Smectite (swelling) clay.