Page 254 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 230

[ ] Well Programming
2.5.6

stage. Finally filtration occurs where all solids are filtered out by the
wall cake and only clear fluid invades the formation.
One point about sized calcium carbonate bridging agent—it is
often promoted because it is acid soluble. However, it can usually be
cleaned off the wellbore by back flushing. If an acid treatment is
required to remove it, acidizing should always be carried out under-
balanced. If acidizing was overbalanced, there is a danger that the par-
ticle size of the bridging agent will become smaller as it dissolves. The
smaller particles may invade the sand. Also, acid may not dissolve all
of the remaining CaCO . It should also be remembered that the plastic
3
viscosity of the mud will increase proportionately to the amount of cal-
cium carbonate that is added.
Calcium carbonate is not the only bridging agent that might be
used. Other forms of bridging agents are sized salts used in saturated
solutions and oil soluble resins. The sized salts can be removed by dis-
solution with water. The oil soluble resin, which is not soluble in water,
dissolves in the crude as the well flows. These techniques have been
highly developed and particularly the use of sized salt, the accompa-
nying polymer mix, and the methods of removing the polymers. It is
recommended to review these techniques further with the mud com-
panies or the specialty companies that produce these materials.
Control of solids and use of bridging agents (to bridge on the face
of the wellbore) is important as it minimizes on solids invasion and
also on fluid invasion of the reservoir. For the same latter reason, the
filtration rates of the fluid should be as low as possible. The lowest safe
differential pressures should for similar reason be applied.
Most drill-in fluids are in a nondispersed condition. This is
because dispersed muds carry much more (damaging) low-gravity
solids. At the same time, the use of dispersants such as lignosulfonates
should be avoided. Lignosulfonates, as well as acting on formation
clays, can disperse interstitial clays in the porosity of the reservoir. If
these clays disperse, they may cause plugging or even weaken the
matrix of the rock. It is common in nondispersed drill-in fluids to have
2 to 3% potassium chloride present to protect these interstitial clays.
Avoiding using dispersed muds can show some large increases in
productivity, particularly in marginal fields. One field in Zaire,
increased well productivity from 400 BOPD to 1100 BOPD because of
changing from a lignosulfonate CMC mud to a lime nondispersed

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