Experimental study of the time to restart the flow of a gelled waxy crude in rheometer and pipeline

Abstract

In the offshore oil industry, in situations where crude oil is at the seabed, the low-temperature environment can cause particles to precipitate, and this generates flow assurance problems. If the production stops and there are high molecular weight hydrocarbons (waxes), the crude can form a gel consistency and block the pipeline. To restart the flow, several phenomena are involved: non-Newtonian behavior, compressibility, breakdown of the molecular structure, among others. The time to restart the flow in such conditions is still unknown since the models offer low accuracy and the industry usually overdesigns the system. Here, we show an experimental method to investigate which phenomenon is relevant to the flow start-up of gelled waxy crudes. Our experimental procedure diverges from most experiments in the literature, where the inlet pressure increases continuously. However, in this study we applied pressure and waited for the flow to restart. With this procedure, we do not have to distinguish whether there was an overshoot due to the elastic response or if that pressure was indeed the minimum pressure at such conditions. We also propose a methodology to compare the results from the rheometer and the pipeline, which are not easily comparable, due to different deformation regimes. At all experiments we applied a pressure/stress higher than the apparent yield stress of the gelled crude and investigated the transient behavior during start-up. We also study the influence of aging on the time to restart the flow under constant boundary conditions; we observed that it increases as the aging increases, both in the rheometer and in the pipeline.