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This presentation focuses on inorganic scale deposits as suppose to organic deposits

such as asphaltenes and paraffins.

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This slide provides an indication of the drastic impact that scale can have on the well’s

production.

In this North Sea example, the client experienced a production decline of 28% due to

scale. It is the 2nd highest factor that results in production decline. This is a good

indication to Schlumberger that we need to work with our clients and relevant

Production Chemical companies, in defining appropriate solutions.

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Inorganic scales

• Hard inorganic crystal – although different scales will have a different magnitude of hardness,

when compared to steel, there is minimal contrast. This puts things in perspective for anyone

who has never been exposed to scale. It is important to understand the hardness of scale and

the implications with removal. This will determine whether a mechanical method (scale blaster)

would be required.

• Variable crystal size – the smaller the particles, the easier to dissolve, the larger the particles the

more difficult to dissolve. Scale has crystal structures and clearly defined chemical

compositions; this makes identification possible using common laboratory equipment or

computer prediction.

• Produced from aqueous solutions – simply equation implies, “no water, no scale”. Because

scales are formed from aqueous solutions and well established chemical reactions; prediction is

made possible by analysis of field waters and chemical modeling.

• Soluble in some special formulations – carbonate scales are soluble in acids whereas sulfate

scales are not.

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The two most common scales in the industry are calcite and barite.

• Calcite – Most naturally occurring waters contain some amount of calcium. If we exceed the

solubility limits of the solution, then calcite could be formed. Calcite is easy to remove, but

difficult to predict using analytical software.

• Barite – Easy to detect, but difficult to remove. There are also HSE issues involved with

removing barite; when barite precipitates, it is very common that radium will precipitate as well,

which makes it easy to detect on a gamma ray log. However, disposal becomes a safety issue.

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An essential concept in scale formation is the mineral solubility; mineral solubility

defines the capacity of a fluid to hold a given amount of the mineral under specific

conditions.

Any fluid can be below its capacity for a given mineral under given conditions.

However, changing the conditions may change the capacity. If the amount of dissolved

mineral is greater then the capacity under the new conditions the fluid will precipitate

the solid.

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Solubility is the amount of a mineral that the fluid will hold in solution.

This table show the great difference between the solubility of NaCl, CaS04, CaC03 and BaS04 at

770 F.

Sulfates, though easily formed, are difficult to remove.

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Minerals will precipitate if their solutions become oversaturated; the more

oversaturated the solution, the faster the mineral will precipitate. Calculation of

exact growth rates (kinetics) is almost impossible for real systems because of the

complex mechanism of precipitation.

Mechanistic steps involve two stages - nucleation and growth:

1. Solutions become over saturated because of perturbations.

2. Specific ion diffuse together to form on pairs which may form clusters or

nuclei. The formation of clusters is reversible but the more oversaturated

the solution the more nuclei will form and the longer they will reside.

3. Clusters can spontaneously rearrange into crystallites. This is called

homogeneous nucleation and is very rare.

4. Commonly clusters form preferentially on rough surfaces (casings etc) and

on existing crystals. This is heterogeneous nucleation.

5. Crystallites are imperfect and further rapid growth is at imperfections on

crystal surfaces.

This mechanism has a strong bearing inhibition treatments

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There are three common causes of scale in oil and gas wells.

(a) Incompatible mixing is most commonly encountered and gives rise to carbonate

and sulphate scales. Perturbations to solubility caused by changing the

concentration of the scaling ions is the driving force.

This phenomenon is controlled by inhibitors.

(b) Self scaling is caused by changing the temperature and pressure of a fluid as it

travels along a wellbore.

For sulphates and carbonates this phenomenon is controlled by inhibitors. For halite

formation it can be severe and difficult to control.

(c) The third and least common cause of scale is evaporation in a gas bubble.

This can only be stopped by saturating the gas bubble with water at its point of entry

into the wellbore. It is a serious but uncommon source of scaling.

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The more data we have the better will be our assessment of the scale

problem.

Ideally we need the lot.

Even with imperfect data we can back out the problem.

pH and precise carbonate analyses are critical and often neglected by

clients.