Yield Behavior of Waxy Crude Gel: Effect of Isothermal Structure Development before Prior Applied Stress

The rheological properties of waxy crude oil are dependent upon the shear and thermal histories. It would not be valuable to study the shear effect of waxy crude oil without consideration of the thermal conditions. Currently, researchers study the shear effect of waxy crude oil mainly with different cooling rates or shear temperatures; few studies have been reported about the influence of isothermal structure development on the shear effect. Present studies show that, even at constant temperature below the wax appearance temperature, the characteristics of the waxy crude gels undergo a change with quiescent duration, becoming harder and more stable. In this paper, a specimen of a typical waxy crude oil was held isothermally and quiescently for a time in the range of 0-60 min for structure development at temperatures close to the gelation temperature, followed by the application of constant stress in a wide range of 1-100 Pa at the same temperature. After subsequent cooling to below a temperature of 2 degrees C, the structure strength of waxy crude gel was examined by measurement of the static yield stress. This showed that the structure strength of waxy crude gel increased with isothermal aging, while the reversibility of the gel structure decreased. In cases of prior application of lower stresses (from 1 to 7 Pa), the situation of shorter aging time was easier to get a strain to overcome the critical strain, which represents the start of structure degradation, and hence induces a lower yield stress value after subsequent cooling. In cases where the specimen was prior loaded with higher stresses (from 10 to 100 Pa), the longer the aging time of structure development before prior stress application, the lower the yield stress tested after subsequent cooling.