Effect of pre-shear on structural behavior and pipeline restart of gelled waxy crude oil

Abstract

Gelled waxy crude oil is a soft material with very complex rheological behavior that plays a critical role in one of the most concerning problems in the flow assurance of deep-water petroleum development—i.e., pipeline restart. Because the rheological behavior of gelled waxy crude oil is very dependent on shear history, it may hopefully reduce the pipeline blocking risk if a transitory startup of the pump (known as transitory restart) is implemented during the pipeline shutdown period and causes the nearly gelled oil to flow. The essence of transitory restart is the influence of pre-shear on the structural behavior of gelled oil. In the present work, transitory restart was simulated by a rheometer to study the structural behavior of crude oil after being pre-sheared with and without subsequent cooling. Numerical simulations were carried out to investigate the effect of structural behavior on pipeline restart. The results show that the gelled crude oil structure is incompletely recoverable once broken in the isothermal condition. As a benefit from the incompletely recoverable behavior, pre-shear can reduce the structural strength of gelled oil and cause the structure to break down more rapidly. Based on this mechanism, transitory restart can reduce the minimum pressure difference required for successful restart and save restart time. In addition, it is found that the propagation velocity of the yield cross-section gradually decreases during the restart process rather than always being equal to the speed of sound in crude oil. The attenuation of the propagation velocity of the yield cross-section decreases with increasing structure strength of crude oil and increasing restart pressure.