(568g) Monitoring Yielding Condition of a Viscoelastic Waxy Oil Gel

Waxy oils behave as viscoelastic fluid at low temperatures due to the formation of a crosslinked network of wax crystals. It requires as low as 0.5 wt% of the wax crystals in the oil to exhibit non-Newtonian characteristics and as low as 2 wt% of wax crystals to convert the oil to a viscoelastic gel. During a production shutdown, a waxy oil inside a long subsea pipeline cools to the low ambient temperature and transforms to a viscoelastic gel due to the formation of this complex network of wax crystals. Once this viscoelastic gel forms in the pipeline, restarting the flow at the start-up becomes a challenge. Low dosage chemicals (such as polyvinyl acetate oligomers and polymers or acrylic oligomers and polymers) have been identified to be effective in depressing the gelation temperature by de-linking or modifying the wax crystals. Monitoring the effectiveness of the chemical is a real challenge in mitigating the risk of plugging the pipeline with this gel. Several laboratory and pilot scale techniques have been investigated to monitor the viscoelastic behavior of these gels. One simple technique most commonly used in the field is based on the determination of the temperature at which this viscoelastic fluid apparently stops deforming under its own weight. Measurement of the viscosity as a function of temperature under low shear is another way to assess the integrity of these gels. A controlled stress rheometer is used to form the gel inside both cone-and-plate and concentric cylinder geometry by cooling the waxy oil under no shear condition and determining the yielding behavior of the gels by applying a programmed stress ramp. A model pipeline (gelometer) system is set-up in the lab to simulate the formation of the gel inside a subsea pipeline. A gradual pressure ramp is applied at one end of the model pipeline and the gel yielding is monitored as a function of pressure. Several gel samples with different concentrations of the low dosage chemicals were prepared and tested using the above-mentioned techniques. This paper will discuss a detailed comparison of the experimental data obtained from these techniques. This study has shown that the controlled stress rheometer is the most reliable tool with the high sensitivity and reproducibility as compared to the others. Therefore, the controlled stress rheomter has been recommended for gel monitoring and assessing the plugging risk of the field pipeline during an extended unplanned shutdown.