Scale can form from brine (formation water) as it undergoes changes in pressure and temperature, or where two incompatible fluids are intermingled.

An example is formation water rich in calcium, strontium and barium when mixed with seawater high in sulphate.

The first of these mechanism generally gives rise to carbonate scales, while the second usually produces sulphate scales.

Examples of heavy scale precipitation

 

Scale problems can arise in various circumstances:

• during drilling and well completion, if the drilling mud or completion fluid is incompatible with the formation water
• at the commissioning stage of new injectors, if the injection water is incompatible with the formation water
• during production, when a well starts to produce formation water with the hydrocarbons
• during wellstream processing, when significant quantities of produced water put process equipment at risk
• commingled production, where wellstreams from various formations, reservoirs or individual wells are mixed together, can make matters worse.

Statoil’s research has been aimed at understanding scale formation, using computer programmes to predict its occurrence, and testing chemicals to eliminate or inhibit its development.

A new, patented oil-soluble scale inhibitor (OSI) allows a reservoir formation to be treated chemically just as a well has been completed and before it is brought into full, uninterrupted production.

Known as squeeze on completion (SOC), this procedure has several advantages:

• it allows wells to be chemically injected while the drilling rig is still on site, thereby saving time and money for subsea wells
• it reduces concern about when to squeeze and the risk of early formation and well damage having an adverse affect on productivity
• it effectively extends the lifetime of a well, and thereby postpones redundancy (decommissioning).