OCT COMPANY, INC.

Barium Sulfate (BaSO4) is of special concern because this scale is extremely insoluble and difficult to remove chemically. In addition, NORM (Naturally Occurring Radioactive Material) deposits typically are associated with barium sulfate because radium can easily substitute for barium in this crystal lattice. Barium sulfate is a common problem in North Sea fields where sea water is injected (high in sulfate) into reservoirs containing formation water rich in barium. Radioactive barium sulfate (NORM) is a significant operational problem in the Gulf of Mexico, particularly where produced waters high in barium commingle with those having a high sulfate concentration.  

 

Strontium Sulfate (SrSO4) is rarely found by itself as a scale deposit, but it is very common to have a significant percentage of strontium incorporated into barium sulfate crystals.  

 

Calcium Sulfate (CaSO4) scale is not as widespread as calcium carbonate, but is a significant problem in some areas such as West Texas. This scale may occur in different forms. Gypsum (CaSO4.2H2), the most common form found in the oilfield, is associated with lowered temperatures. Anhydrite (CaSO4) deposits may occur at high temperatures.

 

Iron compounds are complicated because they can take on a number of forms. One possibility is iron carbonate; this is similar to calcium carbonate except that its solubility is much lower. Iron sulfide is common wherever there is sour production and hence hydrogen sulfide in the brine. Suspended iron sulfide is the cause of “black water”. Various iron oxide scales may occur if there is significant dissolved oxygen and any dissolved iron. Preventing air ingress (e.g., in tanks and pumps) is encouraged to avoid the formation of these solids as well as to reduce corrosion rates.  
 

Prediction of Scale Deposition

 

Given the brine chemistry and operating temperature and pressure, one can calculate the tendency of these common scales to form. These methods compare the actual concentration of the pertinent ions with the calculated solubility. If the actual concentrations exceed the theoretical solubility, then the brine is said to have a positive scaling tendency with respect to that scale compound and may form a deposit.  

Calculation methods range from comparing actual dissolved ions with charts, to a simple spreadsheet program, to very sophisticated computer models, such as ScaleSoftPitzer (Brine Chemistry Consortium), OKSCALE, GWB, Multiscale and ScaleChem (OLI).

 

Among them, ScaleSoftPitzer is considered to be more accurate because it uses Oddo-Tomson method which takes pressure, temperature and ionic strength into account. In addition, the method does not require a pH measurement, but calculates the pH based on the amount of carbon dioxide gas and bicarbonate in the water resulting a greater accuracy in calculating the actual saturation index of a water sample, since pH measurements decline in accuracy very quickly after the sample is taken out of its natural environment.

 

Scale Inhibitors

 

The most common classes of scale inhibitors include:

• Phosphate esters

• Phosphonates

• Polymers, especially polyacrylates

The table below summarizes the attributes of each type of scale inhibitor chemistry.

 

 

 

 

 

 

 

These characteristics are related to the chemical structure of each inhibitor type.  

• The phosphate esters have a phosphorus-oxygen bond; this is relatively weak and explains why it has only fair temperature stability.  

• Phosphonates are characterized by phosphorus-carbon bonds and are less prone to degradation.

• The carbon-carbon bonds in polymer inhibitors give the best chemical stability.  

These come in aqueous based solutions that normally contain 20% - 40% active inhibitor compounds.  Commercial products may also contain an anti-freeze component such as methanol if they are designed for cold climates.  Some scale inhibitors are very low pH products; this means corrosion resistant materials must be used for whatever the full strength product contacts.  

 

Scale Inhibitor Selection

 

There are several factors critical to the selection of a scale inhibitor.

System temperature

The phosphate esters often are the most cost effective choice, but are not recommended for applications where temperatures exceed 175F. At this temperature phosphate esters may degrade within a few hours. The phosphonates are good to 350F, and polymers can be used up to 450F. Treatment duration is an important factor for higher temperature applications. Less expensive, less stable inhibitors may be acceptable for short duration, but not for squeeze treatments where the chemical must survive inside the reservoir for months.  

Scale inhibitor residual

It is important in surface treatments to be able to verify that the appropriate amount of chemical is present. The determination of the scale inhibitor concentration is vital in squeeze treatments because this is the only means of determining if enough chemical is being swept from the formation and into the produced water.  

Type of scale

Some inhibitors are specific to a given scale, some are more general. If you have a single, specific scale problem, the correct custom product is a better choice. It is very important to characterize the scale deposit for a proper understanding of the root cause of the scale problem and to design a cost effective prevention program.  

Severity of scale problem

Fewer products are effective if the scaling tendency is high.  

Cost

It is worthwhile to consider inhibitors that encompass a range of price per gallon. The less expensive products may not be the most cost effective if it must be applied at higher dosages.  

pH

Most conventional scale inhibitors perform less effectively in a low pH environment. This can be a problem for the prevention of sulfate scales in conjunction with acid treatments or carbon dioxide flooding for enhanced oil recovery.  

Weather

The pour point of products needs to be considered in cold climates.  

Chemical compatibility

The scale inhibitor must be compatible with other treatment chemicals such as oxygen scavengers, corrosion inhibitors and biocides. Also, the scale inhibitor must be compatible with the water chemistry itself. Some scale inhibitors may react with high concentrations of dissolved calcium and form an undesirable calcium-scale inhibitor precipitate (called a “psuedoscale”).

Application technique

This factor is most important if the inhibitor is to be squeezed into the formation. This requires that the scale inhibitor survive for several months under reservoir conditions, and that the inhibitor can be detected accurately in the produced water. There is a standard method for phosphate esters and phosphonates, but the analytical methods for the polymeric scale inhibitors are less reliable.

 

Scale Inhibitor Selection

 

For large projects it is worthwhile to do a series of laboratory tests to screen candidate scale inhibitor products. A good laboratory testing program discriminates among the best, mediocre, and poor inhibitor products for a particular application.

 

Because laboratory tests can only approximate the field conditions, these test results give only an estimate of the inhibitor dosage required to prevent the undesired scale.  

 

Two common evaluation methods are:

• Bottle tests 

  • The bottle tests involve preparing scaling brine samples with and without various candidate inhibitors. After several hours, the water is analyzed to determine the amount of precipitate formed. Good inhibitors will show little or no precipitation.  

• Dynamic Tube Blocking Tests

  • The second method flows synthetic brines through a length of small diameter capillary tubing. The pressure drop across the capillary tubing is monitored versus time. Scaling conditions are indicated by an increase in pressure drop. Good inhibitors will prevent increase in pressure drop over a longer period of time.