B41oa oil and Gas Processing Section a flow Assurance Heriot-Watt University

C. Oddo-Tomson Method 
The Oddo-Tomson method is another way of predicting the formation of 
calcium carbonate and various sulphate scales by calculating 
SI
values. It is 
valid between temperatures of 0-200ºC (32-392ºF), ionic strengths of 0-4.0, 
and pressures of 1-1380 bar (0-20000 psig). 

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TOPIC 2: Oilfield Scale 
 
 
 
11 
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3 
Saturation indices are calculated for the different types of calcium sulphate 
minerals, including gypsum (CaSO
4
.2H
2
O), hemihydrate (CaSO
4
.1/2H
2
O), and 
anhydrite (CaSO
4
). 
Gypsum is the most common scale former and occurs at relatively low 
temperatures. Above about 100ºC (212ºF) anhydrite is the stable phase. 
However, hemihydrate can form at temperatures ranging from 90 to 120ºC 
(Oddo and Tomson, 1991). The Oddo-Tomson method can also predict the 
formation of barium and strontium sulphate scales. 
Inputs needed are chemical analysis (including calcium, barium, strontium, 
bicarbonate, carbonate, and sulphate ions), temperature in ºF, pressure in psia 
(psig + 14.7) and mole percentage of carbon dioxide in the gas phase or, if 
there is no gas phase, the amount of dissolved carbon dioxide in the water. If 
the amount of carbon dioxide is unknown, but there is an accurate pH 
measurement, the method uses the pH to calculate the saturation indices. 
The Oddo-Tomson method is more accurate than the Stiff-Davis method 
because it takes pressure, as well as temperature and ionic strength into 
account. This empirical model also incorporates corrections for the presence 
of two or three phases (water, gas, and oil). 
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. 
This allows for 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. 
More complicated prediction packages, such as Multiscale (Petrotech), have 
been developed that can both describe the multiphase behaviour of gas-oil-
water systems and the thermodynamic properties of electrolyte solutions. 
In general, and taking into account the limitations of the carbonate scale 
prediction, these prediction packages provide useful information on the 
possibility of scale formation, its possible magnitude and the 
profile 
of the 
problem throughout the lifetime of a particular field.