by Frederyk Ngantung, Chuck Coburn and Georgeta Hategan
February 29, 2016

The combination of a developmental bio-based flow assurance enhancer (FAE) and commercial polymer inhibitors acts to reduce crude oil’s viscosity and pour point temperature. These reductions translate to higher production rates, lower energy costs and reduced frequency of pumping/operation interruptions that save time and money.

Flow Assurance

As crude oil is being removed from its reservoir, its temperature decreases which can result in wax deposition on tubing and pipeline walls. Remediation of these flow-reducing deposits is an expensive problem that often requires shutting down production. For example in sucker rod oil wells, paraffin scrapers are clamped to the strings or rods to remove waxy deposits before rod stretch or breakage can occur. In surface pipelines, waxy deposits can be removed by introducing hazardous solvents or by pigging. Alternatively an environmentally and worker friendly method of wax control is the addition of polymers inhibitors to the crude oil to reduce or eliminate wax deposits. At low levels, these inhibitors modify the crystal structure of the wax, preventing the formation of oil-trapping and 3-D networks that thicken the crude oil. At high levels, however, they can become entangled or precipitate, leading to even more severe pipeline plugging.

So if the polymer inhibitor is added in amounts beyond the optimum level, its benefits cease and pour point (PP) and/or viscosity may increase. Recently, experts have broken through this barrier by combining the polymer inhibitor with a bio-based FAE. Polyacrylate-ethylene vinyl acetate is used as a model polymer inhibitor system. This synergistic blend has been shown to achieve lower pour point temperatures and viscosity values than those achieved using the polymer inhibitor alone at any concentration.

Pour point studies using a West Texas Intermediate (WTI) crude spiked with a C36 distribution pig wax (2.5 percent by weight) resulted in a crude oil with a PP of -3 C. While the addition of only the FAE (600 parts per million [ppm]) failed to lower the PP, the sole addition of a commercially available polyacrylate-ethylene vinyl acetate inhibitor (PA-EVA, 150 ppm, 50 percent by weight active) reduced the crude oil’s PP to -5 C. Yet, the addition of the FAE (600 ppm), combined with the PA-EVA polymer inhibitor (150 ppm), lowered the oil’s PP to -12 C. Achieving the same level of performance without the use of the FAE required six times the amount of inhibitor (900 ppm) (see Figure 1).

Pour point (PP) study of FAE with PA-EVA-based polymer inhibitorFigure 1. Pour point (PP) study of FAE with PA-EVA-based polymer inhibitor (Graphics courtesy of Elevance Renewable Sciences)

Rheometry Studies

Rheometry studies also confirmed the synergistic effect of the FAE and the PA-EVA polymer inhibitor. While the addition of the FAE to the pig wax-spiked (2.5 percent by weight) WTI crude oil had no effect on the crude’s viscosity, low levels of the PA-EVA polymer inhibitor (150 or 600 ppm) reduced the viscosity and higher levels (900+ ppm) increased it. Combining the FAE with the polymer inhibitor afforded economically advantaged blends and viscosity values unattainable by any concentration of inhibitor by itself (see Figure 2).

Rheometric study of the FAE with a commercial PA-EVA-based polymer inhibitorFigure 2. Rheometric study of the FAE with a commercial PA-EVA-based polymer inhibitor

Rheometry studies were also conducted on the pig wax-spiked WTI crude (2.5 percent by weight) using a modified polycarboxylate polymer inhibitor. In addition, a separate study was conducted and on a WTI crude oil spiked with a C30 distribution wax (10 percent by weight) and a PA-EVA polymer inhibitor. In both cases, the combination of the FAE and the polymer inhibitor showed a reduction in the crude oil’s viscosity when compared with the polymer inhibitor alone. Finally, the other solvent systems such as methyl soyate or aromatics with the PA-EVA polymer inhibitor failed to show this level of viscosity reduction.

Mississippi Crude

Cold finger studies, conducted on a waxy Mississippi crude oil, analyzed the composition of the deposited wax using high-temperature gas chromatography (HTGC). While the deposited wax from the oil spiked with the FAE or with the paraffin inhibitor had similar compositions as the blank, the combination of the FAE and the polymer inhibitor afforded a wax deposit with a much higher ratio of non-normal to normal paraffins (see Figure 3).

HTGC analysis of deposited wax from Mississippi crudeFigure 3. HTGC analysis of deposited wax from Mississippi crude

This elevated amount of non-normal paraffins (branched or microcrystalline) is known to soften the resulting paraffin1, which could translate into easier mechanical or solvent removal. In another application, the FAE can be blended with various solid polymer inhibitors to reduce their viscosities, making their delivery to subsea wellheads less problematic and more effective (see Figure 4).

Addition of FAE to lower a commercial PA-EVA-based polymer inhibitor’s viscosityFigure 4. Addition of FAE to lower a commercial PA-EVA-based polymer inhibitor’s viscosity

The introduction of FEA with a polymer inhibitor either into sucker rod casings, tubings, pipelines, or other oil field flow assurance applications should translate into higher production rates, lower energy costs, reduced frequency of operational interruptions and increased reliability that will save time and money.

Test Methodology

Figure 1: ASTM D97. HCP 852 Automated Pour and Cloud Point Tester. Start temperature 9 C above expected PP temperature and evaluated every 3 C.

Figure 2: A Discovery stress control rheometer DHR-2 (TA Instruments) with 40 mm parallel plates at a 1 mm gap was used to measure viscosities. Aged WTI Crude Oil (flash point >38 C) was spiked with 2.5 percent by weight pig wax (Gaussian distribution of C36) to simulate a waxy crude. Its viscosity as a function of temperature was measured and lowered by the addition of the polymer inhibitor and/or FAE.

Figure 3: Stirred crude oil (60 C) deposited residue onto a cooled copper tube (4 C) over a one hour period. The residue was collected and separated to afford a wax fraction that was analyzed by HTGC.

Figure 4: The same rheometer DHR-2 was used to measure the polymer inhibitor diluted with FAE at different ratios to afford multiple solutions. Their viscosities were measured as a function of temperature.

Reference
1. NIIR Board of Consultants & Engineers, Wax and Polishes, 565-566 (2011)