Yield Stress Measurement and Thixotropic Behaviour of Waxy Crude Oil from the Malay Basin

Abstract

Abstract Predicting the restart pressure for gelled waxy crude has always been a challenge due to the thixotropic nature of the crude apart from the thermal shrinkage and presence of gas voids within the gelled crude. Thixotropy is a phenomenon where the viscosity decreases under constant shear rate followed by a gradual recovery over time when the shear rate is removed. In pipeline transportation of crude oil, for instance, it is important to characterize the thixotropic behaviour of the crude oil in order for the flow characteristics to be well predicted and understood. Various methods have been proposed to quantify thixotropy, including the well-known "thixotropic-loop" tests. Yield stress is the stress corresponding to the transition from elastic to plastic deformation, the value of shear stress above which the material flows. Existing method defines two yield stresses, one static and one dynamic. While static yield stress is more difficult to measure since most occur at very low stresses and that the measuring methods is still subjected to many debates whereas the dynamic yield stress is relatively easier to be obtained and requires linear extrapolation of viscosity data at high shear rates. Therefore, the measurement for static yield stress was done with oscillatory testing and dynamic yield stress with steady state peak hold testing. All measurement was done under the rheometer with minimized wall slip effect for both statically and dynamically cooled sample. In this paper, the thixotropic behaviour of a waxy crude oil from the Malay basin is explored together with the effects of temperature. Data showed that the extent of thixotropy increases with the reduction in temperature below the Wax Appearance Temperature of the crude oil. For the yield stress measurement, it also exhibited some interesting features of the waxy crude oil. The outcome of the study could then be used to fully understand the nature of the gelled crude.