Scale in pipelines, wells, injection manifolds greatly impacts the facility in a number of ways including deferred production, lost revenue and equipment failure.
There are two type of typical scales:
- Calcium carbonate scale is usually located in the top of the wells from 2000 ft and up. It looks similar to the scale you see around the heating element in your home water heater. The calcium carbonate scale has a white-creamy color.
- Barium Sulfate is typically located deep down in the horizontal part of the well below 6000 ft. It has a more yellow to brownish color compared to calcium carbonate scale.
Note: some calcium carbonate scale issues also depends on fluid characteristics, pressure, temperature, bubble point, CO2 concentrations, pH, bicarbonate levels etc. Some scale colors also depends on purity and if other scales co-precipitate alongside. Also, it is not possible to identify in general, the position of scale precipitation without detailed well information. The above only describes the typical scales found in the system.
Possible methods of removing scales:
- Acid washing is an effective removal method of calcium carbonate scale.
- Coil tubing is used to where acid wash is not working. This method has so far been the most effective way to remove barium sulfate scale located deep in the well.
Note: Acid washing will not remove Barium Sulfate scale. Mechanical removal will be required for Barium Sulfate scale.
To prevent scale:
- Gas lift scale inhibition is when a scale inhibitor is continuously injected into the lift gas which then is injected into the well in order to prevent calcium carbonate and iron sulfate scale from depositing on the tubing wall.
- Scale Squeeze Treatment is when a scale inhibitor is bull headed down deep in the well in order to prevent barium scale from forming in the horizontal section of the well.
The more mature the fields are, the more water you will need to squeeze the oil out of the reservoir. It is the water that contains ions such as calcium, barium and sulfates which form scale. The scale types that are experienced in the wells are calcium carbonate, iron sulfides and barium sulfates.
Acid wash, where acid is pumped into the well to remove the scale, is one of the oldest and most used methods. However, the acid only dissolves the calcium carbonate and iron sulfide scales; it does not prevent scale from forming. Acid wash is an expensive mitigation measure as production has to be shut in while the operation is being carried out plus the handling of the acid is a potential safety risk.
There is no single solution to the scale issue. In some places it might be best to wash the calcium carbonate and iron sulfide scale deposits away with acid, in other wells a scale inhibitor has to be applied.
ClearWell Technology is a method where an electric current is created in the tubing and flowline of the well. This current is created by a device mounted externally at the surface. The current should prevent the ions from precipitating out as scale in the tubing – instead the ions are kept in solution and are produced with the well fluids.
Use scale inhibitor to prevent new scale being built up
The scale squeeze treatments have been applied by use of "bullhead treatment" where the scale inhibitor is mixed with the injection water and bullheaded into the well. The well is then closed in for a 6 hours soaking period.
Following the soaking period, the well is then put on production and the scale inhibitor is slowly being produced out of the well with the produced water.
The scale inhibitors performance/efficiency is then monitored in terms of residual scale inhibitor concentration in order not to reach the minimum inhibitor concentration where limited inhibition occurs and where a high potential for scale precipitation exists. Furthermore, ion analysis and ESEM are conducted in conjunction with the RSI analysis in order to assess and evaluate the scale mitigation/development in the well.
Sometimes it is necessary to conduct several scale squeeze treatments on a well in order to prevent further barium sulfate scale precipitation.
Caliper surveys are conducted prior and following the scale squeeze treatments in order to evaluate the efficiency of the scale inhibitor (scale precipitation and corrosion development).
Chemical suppliers like Champion Technologies and Clariant have several scale inhibitors targeting barium sulfate scale for squeeze applications.
It is essential to conduct a static adsorption test and coreflood study in order not to cause any reservoir impairment when conducting the scale squeeze treatments.
Furthermore, ensure that corrositivity of the tubing material and elastomer compatibility have been conducted prior to application of the scale inhibitor.