Paraffin Deposition Problems in the Oilfield
Wax deposition is one of the chronic problems in the petroleum industry. The various crude oils present in the world contain contents of up to 32.5%. Paraffin waxes consist of straight chain saturated hydrocarbons with carbon atoms ranging from C18 to C36. Paraffin consist mostly with normal paraffin content (80-90%), while the rest consists of branched paraffins (iso-paraffins) and cycloparaffins. The sources of higher molecular weight waxes in oils have not yet been proven and are under exploration. Waxes may precipitate as the temperature decreases and a solid phase may arise due to their low solubility.
For instance, paraffinic waxes can precipitate out when temperature decreases during oil production, transportation through pipelines, and oil storage. The process of solvent dewaxing is used to remove wax from either distillate or residual feedstocks at any stage in the refining process. The solvents used, methyl-ethyl ketone and toluene, can then be separated from dewaxed oil filtrate stream by membrane process and recycled back to be used again in solvent dewaxing process.
Removal of Paraffin Deposits
Paraffin accumulations are removed by methods that can be broadly placed into three categories:
- Mechanical removal of paraffin deposits
- Use of solvents to remove paraffin deposits
- Use of heat to melt and remove the wax
Mechanical methods such as scrapers, knives and other tools are most commonly used to remove paraffin deposits in the wellbore. They can be very effective and are relatively inexpensive.
The most common solvent used to remove paraffin from tubulars and the near-wellbore region is crude oil. Hot oiling is the least expensive method, commonly used on stripper wells to remove paraffin deposits. Lease crude taken from stock-tank bottoms is heated to temperatures of 300 degrees F or more. This heated oil is then injected or gravity fed into the tubing or annulus (more common). The high temperature induces solubilization of the paraffin deposits in the injected crude, which is then produced back to the surface. Hot oiling has been used successfully to remove paraffin deposition but can result in formation damage. The use of hot sale water to melt the paraffin may be a safer approach.
Solvents, both organic and inorganic, have been used in the past. These include crude oil, kerosene, diesel and surfactant formulations that can solubilize the paraffin. Organic solvents that consist of a blend of aromatics are usually used to remove mixtures of paraffin and asphaltene deposits. However, the cost of such treatments can be significantly higher than that of hot oil or water treatments.
Steam has been used in a number of cases in which severe paraffin problems have resulted in plugged tubulars. The lack of solubility of paraffin in hot water necessitates the use of surfactants with steam or hot water so that the melted paraffin can be removed.
Methods of Preventing Paraffin Deposition
Several mechanical adjustments can be made in the production string that can minimize the likelihood of paraffin deposition. In general, these steps are designed to minimize the cooling of the crude oil as it is produced to the surface. This can be accomplished by designing pumping wells or tubing sizes and gas lift systems that maximize the flow of oil to the surface and minimize the heat lost to the surrounding formations. Use of more expensive methods such as plastic coatings on tubulars and electrical heaters is severely limited by economics.
Paraffin inhibitors are a class of compounds that consist of crystal modifiers that prevent the deposition of paraffin onto the pipe surfaces. These surface-active materials retard paraffin deposition by inhibiting the adhesion of paraffin to sites on the tubing walls. Surfactants used in these applications include wetting agents, dispersants and crystal modifiers. Each of these chemicals need to be tested for a specific crude oil to evaluate it effectiveness.