Page 183 - Handbook Of Multiphase Flow Assurance

Scale 179

Solubility of sulfate scale in water increases with temperature, so more calcium sulfate de-
posits at lower temperatures. However, the brine instability or scaling index of sulfate scale
may increase with temperature.
Barium sulfate or barite can form downhole in reservoir, wells, in flowlines and in topsides
process equipment. Solubility of barite scale usually increases with temperature, so more bar-
ite can deposit at a lower temperature.
Barite forms when ions from formation water and from injected seawater combine. Barite
scaling index is usually the highest in the 40–70% seawater range.
Sulfate scale deposition can be very rapid. In one example offshore North Sea, production
system got completely blocked in 1 day after the seawater breakthrough, without scale miti-
gation (Kelland, 2014).

Analysis
Scale laboratory tests generally fall in one of the two categories: static bottle test and dy-
namic tube plugging test.
The static test is used to determine the effectiveness of scale inhibitors with limited labo-
ratory equipment. Brines used in the test can represent formation water if carbonate deposits
are studied or a blend of formation water and seawater if sulfate deposits are studied.
The brines are made up of a cationic solution and anionic solution. Chemical inhibitor is
usually added to the anionic solution. The brines are maintained at a fixed temperature and
atmospheric pressure to simulate the production system thermal condition. After a set time
such as 12–16 h, samples are collected from each test bottle liquid phase to find the desired
cations' concentration using titration, inductively coupled plasma atomic emission spectros-
copy (ICP-AES) or atomic absorption spectroscopy (AAS). Absence of ions indicates that they
have precipitated as scale. Relative ability of chemicals to retain ions in solution is propor-
tional to chemical effectiveness for a given brine at a given temperature.
The dynamic tube plugging test uses a blend of saturated brines injected into a tube main-
tained at a set pressure and temperature. The dynamic test can be used to measure the effec-
tiveness of chemical inhibitors in prevention of precipitation, adhesion and accumulation
of deposits inside steel tubes in flowing conditions. The required solutions are prepared by
dissolving salts in distilled water, same as before a static test. Two complex solutions are pre-
pared in order to keep deposit forming cations (solution 1) separate from the deposit forming
anions (solution 2), so that to get the desired brine when the two are blended. It is desirable
that both solutions would be charge-balanced and have the same ionic concentration. This is
accomplished by the adjustment of NaCl content in the solutions.
Special considerations are made when preparing solutions for sulfate deposits such as the
sequence in which salts are dissolved in each solution.
A pump is used to flow the blend of two solutions through the tube, and pressure dif-
ferential is monitored to compare the effectiveness of chemicals relative to a brine without
chemical additives.

Prediction

Scale precipitation prediction is based on the analysis of combined solubility of all ions
soluble in water. Several commercial models are available for scale stability prediction.