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.
Crude oils may contain from 2 to 15% paraffin which may solidify at temperature between 60 to -20C. Paraffin are composed of hydrocarbons containing up to 40 carbon atoms connected in straight chains (normal paraffin) or ramified (iso-paraffin) or chains with a cycle at the end (cyclo-paraffin). The formation of paraffin is favored by:
The reduction in pressure that lead to cooling and loss of light fractions which are the best natural solvent for paraffin
- Contact with cold and rough walls
- Laminar flow regime
- Presence of inorganic deposits
The deposits are treated by mechanical means such as pigging, by thermal treatment such as circulation of warm liquid, or by chemical treatment such as addition of solvent and/or dispersant. Solvents are typically used when paraffin deposits have already formed. They are left in contact with the deposit for several hours in order to dissolve paraffin before pigging is used. The dispersants have a preventive action by decreasing or eliminating the growth of the crystals. They are macromolecules with a structure very similar to paraffin except that they have a polar group at the end of the carbon chain. Their similar structure makes them participate and even initiate the crystallization process. However, the polar groups located at the end of the chains will create repulsion forces which hinder the growth of crystals. Thus, the insoluble paraffin stays dispersed in the crude oil in the form of colloids.