(167a) Effect of Reheating on the Flowability of Waxy Crude Treated with Pour Point Depressant

Waxy crude oils usually present high pour points and poor flowability at ambient temperature, causing significant flow assurance challenges in oil production and transportation. Addition of pour point depressant (PPD) is known as an effective approach to improve the cold flowability of waxy crudes. It is recently discovered that the efficacy of PPD as determined by conventional pour point tests depends on the thermal history of the treated crude oil during the test.

This paper is dedicated to investigating the effect of sample temperature manipulation, especially the reheating operation after PPD treatment, on the measured pour point of treated crude. To achieve this goal, three crude oils A, B and C, treated with 100 ppm of a PPD at temperatures sufficiently higher than their corresponding wax appearance temperatures, were equilibrated at room temperature, reheated to a temperature in the range from 30 to 70 °C, and eventually cooled for pour point determination.

We discovered that reheating the treated sample to a temperature (T_reheat) between the wax appearance temperature (WAT) and the wax dissolution temperature (WDT), i.e., WAT <T_reheat< WDT, causes the efficacy of the PPD to diminish, while reheating to a temperature outside this range, i.e., T_reheat< WAT or T_reheat> WDT does not significantly deteriorate the efficacy of PPD.

When the three treated oils are subjected to reheating to a temperature in the undesired range between WAT and WDT, the measured pour points of the treated oils are 7 °C, 8 °C and 7 °C higher than the corresponding lowest pour points achievable with the same treatment. In the worst-case scenario, reheating can completely nullify PPD treatments. The deterioration of PPD efficacy due to undesired reheating operation is also corroborated by increases in the viscosity and yield strength. In addition, after undesired reheating, the wax particles precipitated upon cooling form an entangled matrix featuring low fractal dimensions and small crystal sizes. This wax microstructure differs from those observed after treatment but in absence of improper reheating while resembles the microstructure of wax particle network generated with an untreated oil.

Based on this investigation, we recommend that caution be exercised when assessing the efficacy of PPD’s with conventional pour point test methods. Special care is required when the reheating temperature of 45 °C defined by the ASTM D5853 method falls in the range between the WAT and WDT of the oil of interest, as reheating to 45 °C will result in diminished efficacy of PPD under this circumstance.

This paper is supported by the National Natural Science Foundation of China (No.51534007, No.51134006 and No.51304224).