Flow Assurance (1)

Produced Water Effect

 

Produced water comes from reservoir. Water is good solvent; therefore it can solve many chemical compound and gas in formation. Water also contains suspended solid and contaminant. In formation, water and chemical compound usually is in equilibrium. When produced water move from formation to surface, due to temperature and pressure change, equilibrium is disturbed. Some chemical compound undissolved, precipitated, and form scale. When produced water contact with pipeline wall which made from carbon steel, produced water can dissolve metal and make corrosion problem in pipeline. When produced water and gas flow simultaneously in pipeline at certain pressure and temperature, they can form hydrate. The hydrate can plug pipeline.  Produced water existence can make problem of flow assurance in pipeline.

Important ions regarding flow assurance are below:

Anion

• chloride Cl^-
• sulfide HS^-
• sulfate SO₄^2-
• bromide Br^-
• bicarbonate HCO₃^-
• carbonate CO₃^2-

Kation

• natrium Na⁺
• kalium K⁺
• calsium Ca²⁺
• magnesium Mg²⁺
• strontium Sr²⁺
• barium Ba²⁺
• iron Fe²⁺ dan Fe³⁺
• aluminum Al³⁺

Cation and anion can combine to form several compounds. When pressure and temperature is changed, solubility of each ion can be changed. Ion can be precipitated from produced water and form solid, such as scale. For example, calcium ion and carbonate ion can form calcium carbonate scale.

Ca²⁺ + CO₃^2- à CaCO_3(s)

Barium ion and sulfate ion can form barium sulfate scale.

Ba²⁺ + SO₄^2- à BaSO_4(s)

Produced water with soluble salt is good electrolyte. It can cause corrosion. When produced water contact with pipeline wall, corrosion will occur. The greater amount of salt or ion in produced water, the more conductive produced water, and the more severe corrosion occurs.

When gas contact with produced water at certain pressure and temperature, hydrate may occur. At high pressure and/or low temperature, hydrate can occur and plug the pipeline. Once pipeline plugged by hydrate, it is require weeks even months to dissociate hydrate. Plugging by hydrate is one of main problem in pipeline, especially subsea pipeline.

Produced water can change characteristic of multiphase stream in pipeline and cause slug. If gas quantity is small, while liquid amount is much, it is hard for gas to “carry” liquid. Therefore, slug will be occurred.

Source : Offshore Pipelines, Boyun Guo, Shanhong Song, Jacob Chacko, Ali Ghalambor, Gulf Professional Publishing, Oxford, 2005