Paraffin (Wax) Deposition

This unit is about the paraffin components found in crude oil and the waxes those components form. Ever since the production of crude oil began, paraffin deposits have been a problem in systems that produce oil with a high percentage of paraffin. Over time, millions of dollars have been spent in research on methods for preventing or removing paraffin deposits. And, millions have been spent on programs to prevent and treat systems plagued with paraffin deposition problems.

What are paraffins?

Paraffins (CnH2n+2) are naturally-occurring organic molecules composed of carbon and hydrogen, which means that all crudes contain some paraffin components. Structurally, paraffins are chains of carbon atoms bonded to hydrogen atoms. Crude oilscontain three types of paraffins:

• Straight-chain
• Branched-chain
• Cyclic

Crude oil that has a high percentage of paraffins is considered paraffinic crude. Although all three types of paraffins can contribute to production problems, this unit will focus on the straight-chained variety. This is because straight-chained paraffins with 16 or more carbon atoms (≥C16) forms a waxy solid at temperatures 68°F (20°C ) and below. Wax is the main reason paraffins can cause problems in production systems.

What’s the culprit: paraffin or wax?

Specifically, waxes cause the problems, but since they are a product of paraffins you will often hear the terms together.

What kind of problems do waxes cause?

When waxes precipitate out of the oil and deposit, they can cause flow problems. They can reduce the flow of production fluids or, in some cases, cause the well to be shut down.

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Sites where wax can deposit include:

• Near wellbores
• Tubing and flow lines
• Chokes
• Separators
• Bottoms of stock tanks

Some wax-related problems also may occur without deposition. For example, waxy oil may become highly viscous as it travels through trunk lines, slowing the flowing. Worse yet, the oil may gel and no longer be capable of being pumped. Waxy crude oil may also lead to poor separation of oil from gas and of oil from water.

What causes these problems?

In the reservoir, paraffins are assumed to be in solution. When crude moves into the wellbore, however, two changes occur that favor paraffin deposition:

Temperature Drops

The solubility of straight-chain paraffins (the ones with higher carbon numbers) decreases as the temperature of the crude drops. In the cooler crude, they can come out of solution and form crystals.

The actual temperature at which the first wax crystal comes out of solution is called the cloud point, or wax appearance temperature. The cloud point is an important data point when it comes to wax deposition, but it’s a data point that can change as other conditions in the well change.

Pressure Drops

When in solution, light end hydrocarbons (less than C16), along with other liquid crude components, act as a solvent for the heavier, waxy hydrocarbons (greater than C16). This solvency helps to keep those heavy hydrocarbons in solution.

As pressure drops, however, the light end hydrocarbons will come out of solution. With the loss of solvency provided by the light end gases, the wax appearance temperature of the crude changes and allows the waxy molecules to precipitate sooner than they would if the pressure had not dropped.

When the heavier (C16+) hydrocarbons precipitate out of the oil, it’s possible they will deposit and cause problems. In most cases, however, there must be deposition before a problem will occur.

Deposition is usually driven by a temperature variation. For example, let’s say that you have crude oil that is 100°F and a pipe wall that is 60°F. As the pressure drops and the light end gases precipitate, the heavier paraffins will come out of solution as well. Once they are free in the oil, they tend to want to migrate from the warmer crude oil to the colder pipe wall, where they will deposit. Besides temperature and pressure, other factors that influence the agglomeration and deposition of wax crystals include:

• Foreign matter in the crude, such as sand, scale, and corrosion by-products (i.e. – iron sulfide), which serve as nuclei for paraffin deposition
• Rough, porous, or irregular surfaces

How can you tell if you have a wax problem?

On site, you may spot a paraffin problem if:

• Well requires frequent “hot oil” treatments.
• System experiences reduced production.
• Flowlines and/or vessels have high back pressure.
• Laboratory analysis of oil sample.

Laboratory analyses typically look for:

• Wax content of the oil
• Distribution of the carious hydrocarbons by their “C” numbers
• Pour point (the temperature at which the liquid will no longer pour)
• Cloud point/wax appearance temperature
• Asphaltene content
• Viscosity

Other laboratory tests using highly-specialized equipment can predict when and where wax crystals are likely to deposit.

What can you do about wax deposits?

Wax deposits can either be remediated (removed) and they can be prevented. There are several remediation techniques available to operators. For wells and flowlines theyinclude:

• Mechanical

o Scraping

o Coiled Tubing

• Thermal

o Hot Oiling

o Hot Water

o Continuous Application of Heat

• Chemical – Wax Solvents/Dispersants

o Batch Treatments

o Continuous Treatments

o Combination Treatments with Mechanical or Thermal Techniques

Of the techniques listed, Champion offers only the chemical solution via its line of Surfatron® and Flotron® dispersants and solvents. These products are formulated to remediate wax deposits just as their chemical classifications suggest:

Dispersants attack the wax crystals causing them to disperse, or scatter, so that they cannot agglomerate and deposit. Solvents, on the other hand, actually dissolve the wax crystals, placing them back into solution and preventing them from forming deposits.

How do paraffin inhibitors work?

When applied to the crude coming from the well, Champion’s Flexoil® paraffin inhibitors disrupt the growth of wax crystals and prevent them from growing large enough to cause troublesomedeposits.

Like most chemical paraffin inhibitors, Flexoil® products are polymeric chemicals that include a waxy (paraffinic) component (see graphic on following page.)

You also may hear paraffin inhibitors referred to as:

• Wax crystal modifiers
• Pour point depressants
• Cold flow improvers
• Viscosity reducers

File:How Paraffin Inhibitors Work.png

How are these products applied?

Solvents are applied as batch treatments. Paraffin inhibitors are continuously injected. Dispersants can be applied by either method.

Batch Applications

Batch applications are used for removing deposits (remediation). This typically involves the use of solvents and/or dispersants or, in some cases, a combination of the two products. Remediation batch treatments are applied down the backside of the well (in the annulus) and in the flowline.

For batch remediation programs to be successful in keeping the system free of wax deposits they must be applied according to a schedule that considers how long the chemical will be effective and how strong is the tendency for wax to precipitate and form deposits. In cases of severe wax deposition, batch treatments often work best when allowed to soak in the well.

Batching solvents:

• Dissolve the wax crystal from the outside in
• Can be used with heat and/or agitation
• Can be used periodically with a continuous paraffin inhibitor program
• Can be used as carriers for dispersants

Batching dispersants:

• Used when > 20 % water is present
• Treated crystals repel other wax crystals and metal surfaces

Continuous Injection

Continuous injection is an effective application method for both dispersants and paraffin inhibitors.

Inhibitors should be applied above the cloud point or wax appearance temperature of the crude. The injection rates are generally determined by the severity of the problem (usually start high –500 - 1000 ppm– and work down.)

How do you know if they’re working?

The effectiveness of a wax treatment program is evaluated by monitoring the same indicators that tell you there may be a problem in the first place. They include:

• Reduced production rates
• Pressure/temperature changes
• Deposit buildup in tubing and flowlines
• Pig runs that collect waxy materials