Novel Polymer Modifications Lead to Next-Generation Pour-Point Depressants
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Current logistics and pipeline-infrastructure limitations make transportation and production of waxy crude oil challenging, necessitating a step change in the chemistry required to mitigate crude-oil-composition issues. For wax prevention, pour-point depressants (PPDs) or wax inhibitors usually are used. A new approach makes use of a special hydrophobe modification in typical polymer systems that significantly broadens the systems’ applicability.
Several wax-control strategies are available in the oil industry. Waxy crude oils are sometimes mixed with a diluent to avoid deposits. The diluent can be a gas condensate, natural-gas liquid, or a lighter crude with a lower wax-appearance temperature. Other possibilities include heating pipelines and isolations, mechanical removal by pigging and wireline cutters, ultrasonic techniques or magnets, microbial treatments or wax dissolvers, wax inhibitors, PPDs, and dispersions. Many polymer compounds are described as PPDs; the ones most extensively used for crude oils are ethylene-vinyl-acetate (EVA) copolymers, poly-n-alkyl acrylates, methacrylate copolymers, and styrene-maleic anhydride copolymer esters. PPD polymer systems incorporate into the wax crystal and alter or even disrupt the crystal structures. Polystyrene-maleic anhydride (PSMA) copolymers can be modified easily with alcohols, amines, or other materials, which render them suitable to adjust the PPD performance for crude oil. However, until recently, research has focused on the effect of the polymer additives on the crystal structure of treated waxes in the crude oil and not on the applicability of the PPD itself. The PSMA PPD polymer systems are solids and must be solubilized by suitable solvents in order to be applied under field conditions.
Equipment and Processes
Synthesis. The polymeric backbone of styrene-maleic anhydride was synthesized by radical polymerization of styrene and maleic anhydride. In a second step, the respective copolymer was esterified with a fatty alcohol or mixtures thereof.
The final product contained approximately 40% alcohol copolymer ester in xylene. The copolymer ester is characterized by complete saturation (92%) of the carboxylic groups with heavy alcohol molecules. The general structure of such a PSMA copolymer esterified with different alcohol mixtures is outlined in Fig. 1.
Model Oils. Two model oils were prepared to mimic waxy crude oils with high paraffinic content and different carbon number distributions of the n-alkanes. The model oils were prepared by mixing 14 wt% synthetic paraffin waxes in n-decane.
Alcohols. The styrene-maleic anhydride copolymer was reacted with behenyl alcohol, C24+ alcohol, and mixtures of behenyl alcohol or C24+ alcohol with 2‑tetradecyl octadecanol.
Equipment and Methods
The pour point (no-flow point) is the lowest temperature at which a crude oil stays fluid when it has been cooled under static conditions. The determination of the pour point was made by use of ASTM D5985, Standard Test Method for Pour Point of Petroleum Products (Rotational Method).
The differences in flow behaviors of Model Oil 1 and Model Oil 2 below their pour points from Newtonian to non-Newtonian flow were investigated by using a rotational rheometer. The temperature-dependent viscosity profiles were measured by use of a cone/plate geometry with a cooling rate of
1°C/min and a shear rate of 6 s−1.
The gel strength was obtained by measuring the yield stress of the fully gelled model oils at 4°C (simulating subsea temperature conditions). The model oils were heated to 70°C and held constant for 10 minutes. Then, they were cooled to 4°C with a cooling rate of 1°C/min. To allow complete gelling of the wax/oil gel, the sample was held quiescent for 1 hour before the stress ramp was applied. The shear stress was increased until the sample yielded. The yield stress is defined as the point of shear stress at which a significant increase in the strain was detected.
Results and Discussion
On the basis of previous research on active PPDs, especially for modified PSMA copolymers, mixtures of linear and branched fatty alcohols were applied to improve the performance of the PPDs in pour-point depression and improve their applicability, particularly with respect to solubility and pour point of the PPD solution. A first set of experiments esterified the PSMA copolymer with behenyl alcohol and C24+ alcohol. As expected, the pour point of a 40% active solution increased with the increasing chain length of the esterified alcohol. However, by incorporating 2-tetradecyl octadecanol, a Guerbet-based alcohol, in the polymer, the pour point of those PPD mixtures could be reduced tremendously. For instance, PSMA-1822 showed a pour point of 12°C, whereas a PSMA with 30 wt% behenyl alcohol and 70% 2-tetradecyl octadecanol reduced the pour point by more than 30°C to a final pour point of –18°C. Moreover, the PSMA based on C24+ alcohol showed a pour point of 26°C and could be reduced even more, by 35°C, with the addition of 2-tetradecyl octadecanol to a final pour point of –9 °C.
The 40% active PPD solutions containing 2-tetradecyl octadecanol in its polymer structure were still liquid and flowable at low temperatures. Thus, they are applicable even in cold environments through direct injection without additional heating.
A following step evaluated how much of the polymer can be diluted in xylene or in aromatic solvents with higher flash points.
Fig. 2 above above visualizes the effect of incorporation of 2-alkyl branching into the polymeric structure of PPDs with long alkyl chains. Even highly active PPD solutions stay liquid and flowable at ambient temperatures. The figure shows 70% active PPD solutions based on PSMA-1822 compared with PSMA-1822/2-tetradecyl octadecanol (70/30).
The results show that, besides the pour-point reduction of the PPD solutions, the viscosity of these polymer-modified solutions was also lowered, therefore making highly concentrated products (70% active) applicable throughout a broad temperature range that helps save solvent and energy costs for heated injection. Indeed, the performance of the PPD in the final crude oil is of the utmost importance. Therefore, the PPDs were screened on models oils specially designed to mimic waxy crude oils. The best results (a change of 48°C) were from PSMA-1822/2-tetradecyl octadecanol (70/30) at a dosage level of 1,000 ppm. In this case, a significant improvement in pour-point reduction could be observed.
The flow behavior under quiescent conditions of Model Oil 1 and Model Oil 2 shows that the solubility of the n-alkanes in the untreated model oils at elevated temperatures was high enough to keep them fully dissolved. The model oils behaved as a Newtonian fluid with a low viscosity. If the temperature dropped, the n-alkanes started to precipitate out of the model-oil fluids and build up a gel-network structure. The model oils showed a non-Newtonian flow behavior with a strong increase in viscosity until the fluid stopped flowing at their pour point or no-flow point. All PPD solutions, showing very good performance in pour-point reduction in Model Oil 1, interacted in such a way with the precipitating wax crystals that they could destroy the gel-network structure, and Model Oil 1 kept its Newtonian flow behavior even at low temperatures such as 4°C. In addition, results showed that, by treating Model Oil 1 with 500 ppm of 40% active PPD solutions based on PSMA-1822 and PSMA-1822/2-tetradecyl octadecanol (70/30), the yield stress was reduced dramatically from 1365 Pa to below 1 Pa at temperatures simulating subsea (4°C) and pipeline shutdown conditions.
Model Oil 2 simulates a crude oil containing a higher fraction of long-chain n-alkanes with a higher melting range (53–74°C) and therefore exhibits a higher pour point (40°C). Model Oil 2 could be treated more efficiently with 40% active PPD solutions based on PSMA‑24+ and PSMA-24+/2-tetradecyl octadecanol (70/30) than with PPD solutions based on PSMA-1822 and PSMA-1822/2-tetradecyl octadecanol (70/30). These polymer solutions containing heavy linear alkyl chains in their polymer structure performed well in pour-point reduction by expanding the Newtonian flow behavior toward lower temperatures and reducing the gel strength from 1830 Pa to below 1 Pa at 4°C.
PPDs with mixed (linear and branched) alkyl esters were synthesized and applied to model oils. By introducing a 2-alkyl branching into the polymer structure, the solubility of the polymer in xylene or high-flash-point aromatic solvents could be almost doubled and the final solvent-based PPDs were still liquid at ambient temperature. Their viscosities ranged between 1.3 and 2.7 Pa•s at an active content of 70%. Significant reduction of pour point and gel strength, accompanied by low-temperature viscosity, was observed in two high-paraffinic model oils. These types of highly concentrated solvent-based PPDs enable the oilfield and refinery industry to overcome the challenge of gelling or blocking in the production, transportation, and storage of high-paraffinic crude oils.
Novel Polymer Modifications Lead to Next-Generation Pour-Point Depressants
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