Wax remediation treatments often involve the use of solvents, hot water, a combination of hot water and surfactants, or hot oil treatments to revitalize production. The following methods are used for removal of wax, paraffin, and asphaltenes:
- Heating by hot fluid circulation or electric heating;
- Mechanical means (scraping);
- NGS, nitrogen generating system, thermo-chemical cleaning;
- Solvent treatments;
- Crystal modifiers.
Wax Remediation Methods
Removal of wax by means of a hot fluid or electric heating works well for downhole and for short flowlines. The hydrocarbon deposit is heated above the pour point by the hot oil, hot water, or steam circulated in the system. It is important for the hydrocarbons to be removed from the wellbore to prevent redeposition. This practice, however, has a drawback. The use of hot oil treatments in wax-restricted wells can aggravate the problem in the long run, even though the immediate results appear fine.
This method is only suitable for cleaning a flowline that is not completely plugged. The wax is cleaned by mechanically scraping the inside of the flowline by pigging. The effectiveness of the pigging operation can vary widely depending on the design of the pigs and other pigging parameters. The pigging strategies in subsea systems, and the pigging requirements for subsea equipment, flowlines, platforms, and FPSO design have been discussed by Gomes et al. Coiled tubing is another effective mechanical means used in wax remediation.
A nitrogen-generating system (NGS), introduced by Petrobras in 1992, is a thermochemical cleaning method. The NGS process combines thermal, chemical, and mechanical effects by controlling nitrogen gas generation to comprise the reversible fluidity of wax/paraffin deposits. Such an exothermal chemical reaction causes the deposits to melt.
Solvent treatments of wax and asphaltene deposition are often the most successful remediationmethods, but are also themost costly.Therefore, solvent remediationmethods are usually reserved for applications where hot oil or hot water methods have shown little success.When solvents contact the wax, the deposits are dissolved until the solvents are saturated. If they are not removed after saturation is reached, there is a strong possibility that the waxes will precipitate, resulting in a situation more severe than that prior to treatment.
Dispersants do not dissolve wax but disperse it in the oil or water through surfactant action. They may also be used with modifiers for removal of wax deposits. The dispersants divide the modifier polymer into smaller fractions that can mix more readily with the crude oil under low shear conditions.
Wax crystal modifiers are those chemically functionalized substances that range from polyacrylate esters of fatty alcohol to copolymers of ethylene and vinyl acetate. Crystal modifiers attack the nucleating agents of the hydrocarbon deposit and break it down and prevent the agglomeration of paraffin crystals by keeping the nucleating agents in solution.
Chemicals are available that can be tailored to work with a particular crude oil composition, but tests should be carried out on samples of the crude oil to be sure that the chemical additives will prevent wax deposition. The combined hot water and surfactant method allows the suspension of solids by the surfactant’s bipolar interaction at the interface between the water and wax. An advantage of this method is that water has a higher specific heat than oil and, therefore, usually arrives at the site of deposition with a higher temperature.
Assessment of Wax Problem
The process of assessment for a wax problem can be summarized as follows:
- Obtain a good sample;
- Cloud point or WAT based on solid-liquid-equilibria;
- Rheology: viscosity, pour point, gel strength;
- Crude oil composition: standard oil composition, HTGC;
- Wax deposition rates: cold finger or flow loop;
- Wax melting point;
- Consider the use of wax inhibitors.
Wax Control Design Philosophies
Wax control guidelines for the subsea devices and flowlines can be summarized as follows:
- Design the subsea systemto operate above theWAT by thermal insulation.
- Operate the well at sufficiently high production rates to avoid deposition in the wellbore and tree.
- Remove wax from flowlines by pigging, and pig frequently enough to ensure that the pig does not stick.
- Utilize insulation and chemicals to reduce pigging frequency.
- Identify and treat high pour point oils continuously.
For the pour point the strategies can be summarized as follows:
- In steady-state operation: heat retention (pipeline insulation) to maintain temperatures above the pour point;
- For planned shutdown and start-ups, injection of PPD;
- For unplanned shutdown, focus on restarting the system within the cooldown time of pipeline insulation; if this is not possible, use export pumps to move the gelled plug as early as possible. The required cooldown time has yet to define by operations. In steady and transient states, the strategies can be summarized as follows:
- In steady-state operation, heat retention (pipeline insulation) is used to maintain temperatures above WAT as far along the pipeline as reasonably possible, especially in the deepwater section. Regular operational pigging will be needed throughout life to remove wax deposition.
- In transient operations, wax deposition is considered a long-term issue, so short durations (i.e., during start-ups) of low temperatures will not be addressed.
 ASTM D97-09, Standard Test Method for Pour Point of Petroleum Products, American Society for Testing and Materials, West Conshohocken, PA, 2009.
 ASTM D5853-09, Standard Test Method for Pour Point of Crude Oils, American Society for Testing and Materials, West Conshohocken, PA, 2009.
 Pipeline Deepstar, Wax Blockage Remediation, Deepstar III Project, DSIII CTR 3202, Radoil Tool Company (1998).
 J.R. Becker, Crude Oil, Waxes, Emulsions and Asphaltenes, Pennwell Publishing, Tulsa, Oklahoma, 1997.
 S.E. Lorimer, B.T. Ellison, Design Guidelines for Subsea Oil Systems, Facilities 2000: Facilities Engineering into the Next Millennium (2000).
 B. Edmonds, R.A.S. Moorwood, R. Szczepanski, X. Zhang, Latest Developments in Integrated Prediction Modeling Hydrates, Waxes and Asphaltenes, Focus on Controlling Hydrates, Waxes and Asphaltenes, IBC, Aberdeen, 1999, October.
 Y. Chin, Flow Assurance: Maintaining Plug-Free Flow and Remediating Plugged Pipelines, Offshore vol. 61 (Issue 2) (2001).
 Y. Chin, J. Bomba, Review of the State of Art of Pipeline Blockage Prevention and Remediation Methods, Proc. 3rd Annual Deepwater Pipeline & Riser Technology Conference & Exhibition, 2000.
 M.G.F.M. Gomes, F.B. Pereira, A.C.F. Lino, Solutions and Procedures to Assure the Flow in Deepwater Conditions, OTC 8229, Offshore Technology Conference, Houston, Texas, 1996.
 B. Ellision, C.T. Gallagher, Baker Petrolite Flow Assurance Course., 2001.
 B.T. Ellison, C.T. Gallagher, S.E. Lorimer, The Physical Chemistry of Wax, Hydrates, and Asphaltene, OTC 11960, Offshore Technology Conference, Houston, 2000.
 H. James, I. Karl, Paraffin, Asphaltenes Control Practices Surveyed, Oil & Gas Journal (1999, July 12) 61–63.