As crude oil comes to the surface and exits a well, it cools down, generally causing the wax components of the crude to gel. The gelled crude chokes the well, leading to restricted or blocked production and costly downtime for operators. One of the most common chemical solutions to address the wax deposit challenge is the application of wax inhibitors or pour point depressants (PPDs) to the production stream.

Most of the PPDs used in the field are organic solvent- based. To maintain a stable, free-flowing formulation at very low temperatures, these PPDs need to be heavily diluted down to about 2% to 5%. The solvents of choice quite often are large quantities of toxic organic solvents such as aromatic solvents (e.g., xylene, toluene or methanol). If the proper solvent is not used or the activity of the formulation is too high, the PPD will gel, becoming extremely difficult to handle and dose efficiently. The application of a large amount of solvent is not cost-effective and poses threats to the environment, storage facilities and the safety of the personnel handling the materials.

Qualifying PPD use

To address this challenge, Solvay developed a waterbased PPD dispersion system centered on the company’s polymerization technology. This technology is a specifically designed amphiphilic polymer that is synthesized with a hydrophilic polymeric head group and hydrophobic tail. The molecular weight and, specifically, the architecture and particle size of this polymer can be controlled, creating a unique and stable dispersion. This synthesized product can maintain high activity (15% to 30% or higher) and remain pumpable even under -40 C (-40 F) after properly formulating the polymer with a mutual solvent and wetting or dispersing surfactant. Generally, the viscosity of the PPD, at 40% activity, is between 200 cp and 250 cp at room temperature with a milky color.

There are four common standards for which to qualify a PPD for use. The first is that the product must be thermally stable. Solvay’s water-based PPD is stable up to 200 C (392 F) under 500 psi tested by a Chandler Viscometer 5550. Second, the material should be environmentally friendly. The PPD is dispersed in water or a water-mutual solvent package, which replaces the need for the application of toxic solvents. The third standard is the viscosity of the formulation at -40 C. Solvay’s PPD has a pour point of -30 C (-22 F) at 40% activity and can be formulated to be pumpable under -40 C. Finally, the flash point of the material must be considered as both a concentrate and a formulated product. This PPD is not hazardous in this aspect since it is dispersed in water and/or a high flash solvent.

PPD testing

Once the four standards are met, the deposit control and pour point reduction performance is examined. The performance of Solvay’s first-generation water-based PPD dispersion was evaluated using several crude oils from West Texas. This first-generation PPD was compared to some commonly used solvent-based PPDs, and the results showed that the Solvay PPD significantly reduced crude oil wax deposition by nearly 70% and reduced the pour point of the crude by 18 C (64.4 F). The deposit control performance was evaluated using a standard cold-finger apparatus, and the pour point reduction was tested using a PSL Systemtechnik GmbH pour point tester (Figure 1).

It is well-known that PPD performance is crude-specific. To make an accurate comparison, a model crude oil is blended following a published recipe. The model crude oil comprised a mixture of paraffin waxes (from Sigma Aldrich) dispersed in a low molecular weight hydrocarbon such as decane, dodecane and tetradecane. The low molecular weight hydrocarbon was used as a solvent for the wax and to aid differentiation of the fractions by gas chromatography if required.

The mixture of paraffins (5%) was added to the solvents. The performance attributes of the aqueous polymer dispersions and commercial wax inhibitor/PPD polymers were screened by adding them to samples of 5% weight to volume paraffin wax in aliphatic hydrocarbon. The polymers were dosed at 500 ppm (as active), respectively. The samples were mixed thoroughly and stored at 85 C (185 F) (above the wax appearance temperature) for 1 to 2 hours to eliminate the thermal history of the samples before cooling overnight in a refrigerator at 0 C to 5 C (32 F to 41 F).

The appearance of the samples was noted after removal from the refrigerator and assessed using a polarized light microscope with camera attachment.

The microscope was set at 100x magnification (Figure 2), and the textures observed were viewed either with crossed polarizers or crossed polarizers and mica filter (1/4 λ test plate). Images of the textures were taken using a digital camera and processed using image capture software (Figure 2). The model crude oil gelled up and formed a large amount orthorhombic or needle- shaped (bayonet) crystals (10 μm to 30 μm). The competitive PPD inhibited the formation of large wax crystals, but the size of the wax crystal (about 10 μm) is much larger than that in the water-based PPD dispersion (about 1 μm to 5 μm). Also, the model crude oil has higher viscosity (not flowing as well) compared to the Solvay water-based PPD, indicating a much better performance of the Solvay water-based PPD.

FIGURE 2. This example tested the wax inhibition performance of the water-based PPD in simulated crude oil comparing other commercial PPD products observed by microscope. The simulated crude remained flowable with little wax crystals formed compared to competitor products. (Source: Solvay)

Moreover, compared to the conventional organic PPD, Solvay’s water-based PPD has an advantage in low-temperature applications. An example of a situation where Solvay’s PPD dispersion will have an advantage is in its use in very cold climates. To be able to successfully apply a typical PPD in a cold environment, the PPD would have to be heavily diluted to about 2% to 5% active, using xylene to maintain low viscosity so that it can be dosed into the well. The amount of active polymer actually applied is very limited, and much of the solvent added is wasted simply as a carrier. However, Solvay’s water-based PPD dispersion can remain pumpable at 20% activity, a far greater active level than traditional PPDs and thereby increasing efficiency.

PPD and wax inhibition performance is certainly very crude-specific, as it depends on many particular parameters of the crude, including API gravity, light ends, asphaltenic content and carbon chain distribution to name a few. Solvay is developing a series of these polymeric water-based PPD dispersions to address the many different crude varieties, helping the industry tackle this important flow assurance challenge.

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