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

2.1.1 Acid Soluble Scales 
Acid soluble scales include: 
•
Calcium carbonate (calcite; CaCO
3
) – caused by the presence of both 
calcium and bicarbonate in formation brine. It is by far the most 
common type of carbonate scale. 
•
Iron carbonate (siderite; FeCO
3
) – caused by corrosion products and 
bicarbonates in formation brine. 
•
Iron sulphides (pyrite; FeS, FeS
2
) – caused by corrosion products and 
sulphate reducing bacteria. 
•
Iron oxides of various types (including Fe(OH)
2
, Fe
2
O
3
, Fe
3
O
4
) – 
caused by corrosion products and oxygen-rich injected water. 
2.1.2 Acid Insoluble Scales 
Acid insoluble scales include: 
•
Barium sulphate (barite; BaSO
4
) – caused by barium in formation water 
and sulphate commonly found in injected seawater. 
•
Strontium sulphate (celestite; SrSO
4
) – caused by strontium in 
formation water and sulphate commonly found in injected seawater. 
•
Calcium sulphate (> 110ºC anhydrite: CaSO
4
; < 110ºC gypsum, 
CaSO
4
.2H
2
O) – caused by calcium in formation water and sulphate 
commonly found in injected seawater. 
•
Mixed barium/strontium sulphate ((Ba,Sr)SO
4
) – caused by barium and 
strontium in formation water and sulphate commonly found in injected 
seawater. 
Exotic scale types such as barium carbonate (witherite), strontium carbonate 
(strontianite), calcium fluorite (fluorite), sodium chloride (halite) and 
magnesium hydroxide (brucite) are very unusual scales. These are associated 
with high temperature, high pressure (HT/HP) reservoirs in the North Sea and 
other oil/gas provinces – these uncommon scales will not be covered in this 
course.