Paraffin (wax) deposition is found practically whenever oil is produced and transported, causing damage and therefore lowering production.
Paraffin deposits are typically made up of linear, saturated hydrocarbon chains (C.sub.16 to C.sub.80) admixed to branched hydrocarbons, asphaltenes, water and inorganic compounds such as sand, rust, iron sulfide, clays, etc. The deposit hardness depends chiefly on the oil amounts and mineral substances present in the mixture.
The phenomenon of the deposition or precipitation of solid wax constitutes an example of fluid/solid phase equilibrium, which is explained in the light of solution thermodynamics principles, that is, the solution of a higher molecular weight hydrocarbon in lower molecular weight hydrocarbons which function as solvents. That is, high molecular weight solids precipitate whenever is reduced the transport ability of the fluid solvent.
Wax precipitation is gradual, leading to different chain lengths according to the depth of the paraffin deposit-containing well. The wax deposition mechanism depends on pressure and temperature. Generally, lower pressures tend to increase the cloud point temperature. The cloud point temperature range of several waxy crudes is such that the wax can precipitate even on the formation face as well as within the formation throughout the pressure reduction phenomenon which normally occurs during the well useful life. The lowering in production rate can be wrongly attributed to the reservoir depletion, while many times it is due to the formation permeability reduction and/or to the diameter of the production string.
Hydrocarbon chains of different lengths are present in varying percentages as well as melting points, becoming insoluble at different temperatures. Organic solvents like hot xylene are able to dissolve paraffins and asphaltenes, but not the inorganic deposits; on the other hand, asphaltenes are insoluble in hot pentane.
Removal of the paraffinic deposit is effected through three main methods: thermal, mechanical and chemical, mechanical methods being used specially for production strings and conduits.
The main known thermal method employs heated oil, pumped through the annulus in order to remove the paraffin. This method can be safely used only when paraffin is deposited above the perforations, since heated crude oil can carry paraffin inside the formation where it can cool and settle in the porous spaces.
One alternative to hot oiling is hot water, the higher thermal content of which being able to melt paraffins, although at high cost; besides, hot water does not dissolve nor dilutes the paraffin and can cause emulsion and corrosion problems as well as swelling of a few clayminerals present in the formation. However, paraffin deposits can be chemically dispersed by hot water.
In the mechanical methods, paraffin is physically removed from the wall surface. Surface agents and solvents aid in the removal, paraffin being dispersed in solution.
In the absence of the combined use of solvent and/or dispersants, thermal and mechanical methods simply displace paraffins, where they can cause another problem. Only chemical methods can dissolve or disperse paraffin deposits in oil. Normally, chemical removal is effected through the aid of solvents or surface agents. Paraffinic solvents dissolve deposits; their action is, however, limited to the deposit surface, while surface agents lower surface tension and chemically wrap small particles, thus changing their mutual adhesion capacity, or their adhesion to tubing (pipe) surfaces. Thus the smaller paraffin particles keep oil-suspended and move freely. The presence of a surface agent lowers the water surface tension breaking the link between paraffin molecules and the tube metallic wall, avoiding further paraffin deposition, therefore acting secondarily as a preventive method. Normally, surface agents are diluted in solvents, aqueous or not, and injected in the area showing damage caused by paraffin deposition.