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

2.6 The Impact of Mineral Scale Formation 
The possible areas where scale can form depend on the type of scale 
formation that is anticipated. 
2.6.1 Why and Where Scale Form 
The two types of scale that can form are carbonate scales and sulphate 
scales. The reasons for the different types of scale, and where they are likely 
to be located, have already been discussed – however, these are summarised 
again below: 
Scale Type I (Carbonate Scale) 
 
Why:
Carbonate scale will form when pressure falls below the bubble point of 
CO
2
. At the bubble point, CO
2
is completely dissolved in a saturated liquid 
water phase. Below the bubble point some aqueous CO
2
moves into the gas 
phase, the pH of the aqueous phase rises and, in the presence of Ca, Mg or 
Fe cations, carbonates may be precipitated from solution. 
 
Where: 
During early production carbonate scale may form in tubing, subsea 
safety valves, chokes, etc. Carbonate scale is normally regarded as a problem 
at a lower watercuts (in the range of 10% to 15%.). 
Later in production (or during strong draw down) carbonate scale formation 
can occur within the perforations, in gravel packs, or within the near-wellbore 
formation itself. 

---

TOPIC 2: Oilfield Scale 
 
 
 
28 
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3 
There is less drawdown in horizontal wells compared with vertical wells. Thus, 
in horizontal wells carbonate problems are usually not so significant in the 
formation, but can still be a problem in production tubing and/or topside 
facilities. That is, these problems are generally moved “further uphole” when 
horizontal wells are compared to vertical wells. 
Carbonate scale formation is common from the wellhead and then throughout 
the topside facilities. If the scaling tendency of the brine is low, this may only 
be noted at the discharge pipe where the pressure is very low (10s of psi to 
atmospheric pressure).