Produced fluids may contain insoluble microparticles such as clay, silica, asphaltene in the form of colloidal suspension. However, deposit problems in producing operations are mostly caused by soluble compounds at downhole conditions. These compounds crystallize when the production fluid rises in the well. Salts and paraffin form and grow in preferential areas such as valves, pumps and rough surfaces. These deposits can grow until complete plugging of the well. Reducing or eliminating these deposits early is preferred. If plugged the well requires expensive treatment and production is shut down for several days.
In general, temperature increases the solubility of scales except for calcium carbonate, calcium sulfate and magnesium calcium carbonate. The pressure has a minimal effect on scale solubility. However, pressure changes in the tubing are responsible for many scale formations. In general, the higher the salinity, the higher the solubility of the salts.
Sodium chloride, calcium carbonate, calcium sulfate, barium sulfate and strontium sulfate are the main mineral scale formed in producing operations.
Sodium Chloride – It is formed by saturation and is washed with soft water.
Calcium carbonate – The scale is formed when the pressure decreases, releasing carbon dioxide and changing bicarbonates to carbonates. It is removed by acidification. However, when corrosion is a concern, a scale inhibitor is preferred.
Calcium Sulfate – It is a relatively soluble salt. However, when close to its solubility limit, it can form hard deposits. Decrease of pressure in the tubing lead to partial evaporation of water and result in supersaturation and rapid precipitation of the salt.
Barium Sulfate – In general, deposits of barium sulfate result from water incompatibility. Either incompatible water formations are mixed in manifolds or worse, water injected for secondary recovery is incompatible with the formation water. Barium sulfate is the most troublesome deposit since its solubility is very low.
The evaluation of scale inhibitors may be performed by static or dynamic tests. Laboratory tests need to be conducted with simulated water having a composition as close as possible to that of the field.