Several methods of wax control and management are practiced by production operations, but the transportation of crude oil over a long distance in subsea system demands significant planning and forethought. The wax management strategy generally is based on one or more of the following methods:
- Flowline pigging;
- Thermal insulation and pipeline heating;
- Inhibitor injection;
- Coiled-tubing technology.
The most common method of wax control is flowline pigging if the wax has formed. The solid deposit is removed by regularly removing the wax layer by the scouring action of the pig. Chemical inhibitors can also help control wax deposition, although these chemicals are not always effective and tend to be expensive. In cases where flowline pigging of the production lines is not practical, particularly for subsea completions, wax deposition is controlled by maintaining fluid temperatures above the cloud point for the whole flowline.
Coiled-tubing technology has become an important means of conducting well cleanup procedures. This technology involves the redirection of well production to fluid collection facilities or flaring operations while the coiled tubing is in the well. Heavy coiled tubing reels are placed at the wellhead by large trucks, the well fluids are diverted, and high-pressure nozzles on the end of the coiled tubing are placed in the well. Tanker trucks filled with solvent provide the high-pressure pumps with fluids that are used to clean the well tubing as the coiled tubing is lowered into the well. The value of this method is apparent in many areas of the world, since certain integrated production companies maintain a fleet of coiled tubing trucks that remain busy a large percentage of the time.
Good thermal insulation can keep the fluid above the cloud point for the whole flowline and thus eliminate wax deposition totally. Although line heaters can be successfully employed from the wellhead to other facilities, the physical nature of the crystallizing waxes has not been changed. This can be a problem once the fluids are sent to storage, where the temperature and fluid movement conditions favor the formation of wax crystals and lead to gels and sludge.
The rate of deposition can be reduced by flowline insulation and by the injection of wax dispersant chemicals, which can reduce deposition rates by up to five times. However, it must be emphasized that these chemicals do not completely stop the deposition of wax. Therefore, it is still necessary to physically remove the wax by scouring the flowline. To facilitate pigging, a dual-flowline system with a design that permits pigging must be built. Pigging must be carried out frequently to avoid the buildup of large quantities of wax. If the wax deposit becomes too thick, there will be insufficient pressure to push the pig through the line as the wax accumulates in front of it.
Pigging also requires that the subsea oil system be shut down, stabilized by a methanol injection and blowdown, and finally restarted after the pigging has been completed. This entire process may result in the loss of 1 to 3 days of production. The deposition models created based on the fluids analysis work and the flow assurance calculations are the key to establishing pigging intervals that are neither too frequent to be uneconomical or too infrequent to run the risk of sticking the pig in the flowline. OLGA models the “slug” of wax pushed ahead of the pig.
Chemical inhibition is generally more expensive than mechanical pigging, although the cost comparison depends on pigging frequency requirements, chemical inhibition effectiveness, and many other factors. Chemical inhibitors can reduce deposition rates but rarely can eliminate deposition altogether. Therefore, pigging capabilities still have to be provided as a backup when chemical inhibition is used. The chemicals must match the chemistry of the oil, at the operating conditions, to be effective. Testing of inhibitor effectiveness is absolutely necessary for each application. The tests should be carried out at likely operating conditions. The chemical inhibitors for wax prevention include:
- Thermodynamic wax inhibitor (TWI): Suppresses cloud point, reduces viscosity and pour point, requires high volume.
- Pour point depressants: Modify wax crystal structure, reduce viscosity and yield stress, but do not reduce rate of wax deposition.
- Dispersants/surfactants: Coat wax crystals to prevent wax growth; alter wetting characteristics to minimize wax adhesion to pipe wall or other crystals.
- Crystal modifiers: Co-crystallize with wax, reduce deposition rate, but do not prevent formation, modify wax crystals to weaken adhesion and prevent wax from forming on pipe wall, inhibit agglomeration; suitable for steady state and shutdown, reduce viscosity/pour point, no universal chemicald performance is case specific, high cost, pigging still required, inject above cloud point.
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