(85f) Effects of the Presence of Water in Cold Restart of Waxy Oils

Wang, Y. - Presenter, University of Utah
Deo, M., University of Utah
Magda, J., University of Utah
Recent work has shown the validity of cold flow, a technology to transport waxy crude oils below their wax appearance temperatures (WAT). The thermal flux that results due to the difference between the temperature of the transported oil and ambient is minimized in the implementation of cold flow – thus significantly lowering deposition. Restarting pipelines carrying waxy oil under cold flow is an important consideration. Previous work from this group had shown that restart pressures required for pipelines shutdown under cold flow at a given temperature were lower than pressures required to restart conventionally transported oils (above WAT). In this paper we examined the applicability of this conclusion when large amounts of water were present with the oil.

A model oil consisting of 7% wax and about 0.5 wt% of an appropriate surfactant was used in experiments with water. Maximum amount of water in emulsions was 40%. A reproducible rigorous mixing protocol was developed to create emulsions stable over the course of the experiments. Comprehensive rheological measurements were performed on water-free and emulsified samples using cone and plate geometry. The WAT and gel point were measured using the oscillation method with a slow cooling rate. We used creep methods to measure the static yield stress for gels at 5 °C formed under hot/cold flows. Yield stress measured for emulsion gels is largely lower than that for water-free gels. We studied the effects of roughness of the cone and plate surface and found that higher yield stress was observed using the roughened geometry.

Restart studies were conducted in a bench-scale, 0.4-inch diameter, 4-foot long flow loop. The distinguishing feature of the flow loop was a conditioning loop consisting of a scraped heat exchanger to prepare the cold slurry for restart experiments. Water-free cold flow experiments were first performed to establish a baseline. The measured restart pressures for cold flow restarts at 5 °C when shutdown temperatures were 5-10 °C below the WAT were approximately three times lower than when “hot” restart (cooling from above the WAT to the designated temperature) were used. The restart pressures decreased as progressively lower temperatures were used at shutdown. Restart with emulsions showed that even lower restart pressures were needed for both cold and hot restart. The flow loop experiments results agreed with the rheological measurements that emulsion gel is weakened comparing with the water-free waxy gel. The dependence of lower pressure for progressively colder restarts observed for water-free systems was minimal to non-existent and consistently lower restart pressures were observed. Observations of gelled emulsions under the microscope at different temperatures confirmed the disorganized structure of the gel with water present. We believe this morphology of the gel was primarily the cause of the lower restart pressures observed for emulsified waxy oils.