51
3
Scale Control
3.1 INTRODUCTION
Scale formation is the deposition of sparingly soluble inorganic salts from aqueous solutions.
1
There
is another type of scale containing metal ions in which the anions are organic carboxylates or naph-
thenates. This is discussed in Chapter 7. Scale can deposit on almost any surface so that once a scale
layer is rst formed, it will continue to get thicker unless treated (Figure 3.1).
2
Scales can block pore
throats in the near-wellbore region or in the well itself, causing formation damage and loss of well
productivity. They can deposit on equipment in the well, such as electric submersible pumps or slid-
ing sleeves, causing them to malfunction. Scale can occur anywhere along the production conduit,
narrowing the internal diameter and blocking ow and, nally, scale can form in the processing
facilities. Next to corrosion and gas hydrates, scale is probably one of the three biggest water-related
production problems and needs to be anticipated in advance to determine the best treatment strat-
egy. For some elds, scale control can be the single biggest operational cost.
3
A useful book on scale
control from the operational perspective of a service company has been published.
1
Other reviews
have also been published.
327
3.2 TYPES OF SCALE
The most common scales encountered in the oil industry in order of prevalence are
Calcium carbonate (calcite and aragonite)
Sulfate salts of calcium (gypsum), strontium (celestite), and barium (barite)radium may
also be found in the lattice, especially that of barium sulfate
Sulde scales—iron(II), zinc, and lead(II) salts are the most common
Sodium chloride (halite)
Most minerals are less soluble as the temperature decreases (although calcium carbonate is an
exception, as discussed later). Thus, long-distance, cold water tie-backs can give enhanced pipeline
scale problems as the produced uids cool to seabed temperatures. Furthermore, the deposition
of all these inorganic scales is exacerbated by the presence of organic thermodynamic hydrate
inhibitors (THIs) such as methanol or small ethylene glycols, methanol being worst.
4,328–330
This is
because these solvents are less polar than water and reduce the solubility of common scaling salts.
As more and more elds are being developed in deeper and/or colder water, the use of THIs will
increase to combat hydrate formation, and with this, the number and severity of the scale problems
will also increase. The challenges facing scale control in deepwater elds has been reviewed.
5
Other more exotic scales are sometimes encountered in the oileld. They include iron carbonate
(siderite, mainly from corrosion), calcium uoride (uorspar, as a by-product of HF acidizing),
331
silicate salts,
1,6
and trona (Na
3
H(CO
3
)
2
× 2H
2
O).
7
Silica/silicate scaling is sometimes encountered
in steam ood operations.
8
Methods that may be used to control silica/silicate scaling in steam
ood operations include dilution with fresh water; reducing the pH of the water; treating the water
with reducing, complexing, and sequestering agents; removing silica from water by lime softening;
precipitation of silica in water with metals or cationic surfactants; and, lastly, treating the water
with geothermal silica scale inhibitors/dispersants.
1
A review of factors inuencing silica scaling

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