Page 182 - Handbook Of Multiphase Flow Assurance

178 5. Flow restrictions and blockages in operations

Modeling of reservoir souring
Souring is modeled by commercial models which account for fluid inflow such as from a
water injection well, and temperature distribution in the reservoir between the injector and
producer wells.

Scale

Solid scale deposits form when ions dissolved in water reach a saturation concentration,
and are no longer soluble in water. Due to a great variety of ions, and their ability to combine
as ionic compounds, there are several scales which can be encountered in petroleum produc-
tion systems. Commonly encountered scales are calcium carbonate, calcium sulfate, barium
sulfate and halite.

Description
Scale is composed primarily of insoluble barium, calcium, and strontium compounds that
precipitate from the produced water due to changes in temperature and pressure. Radium is
chemically similar to these elements and, as a result, it is possible that radium can form part of
the scales. The API PUBL 7103 Management and disposal alternatives for naturally occurring
radioactive material (NORM) wastes in oil production and gas plant equipment found that
the highest concentrations of radioactivity are in the scale in wellhead piping and in produc-
tion piping near the wellhead.

Carbonate
Carbonate is among the more commonly occurring scales, and the ones easiest to treat.
Solubility of calcium carbonate scale decreases with temperature, so more calcium carbon-
ate deposits at high temperature. High temperature conditions may exist downhole and at
electrical submersible pumps running dry or hot.
Downhole pressure changes may cause evolution of CO 2 from produced water, which al-
–
ters the content of HCO 3 ion, which in turn leads to the formation of CaCO 3 or calcite scale.
Scaling index (tendency to precipitate scale) for calcite is lower at high pressure and higher
at low pressure.
Calcite scale from formation water can form and deposit in wells and in production flow-
lines. When seawater is injected, calcite can also deposit in reservoir causing formation dam-
age because seawater has very low HCO 3 (bicarbonate) content. If seawater is injected in a
carbonate reservoir with high CO 2 content, its acidity will increase as sour CO 2 dissolves in
2+
–
seawater. Acidic seawater will dissolve carbonate rock and gain Ca and HCO 3 ions. As res-
ervoir fluid approaches producer well, its pressure drops which causes calcite precipitation in
the near-wellbore area and formation damage (Braden and McLelland, 1993; Lu et al., 1997).

Sulfate
Calcium sulfate or gypsum is another scale which can form in wells and flowlines.