Startup flow of elasto-viscoplastic thixotropic materials in pipes

Waxy crude oils transportation is one of the main challenges in offshore oil industry, specially after long shutdown periods when the flow has to be restarted. From the reservoir to the platform, the high-temperature crude oil undergoes a sudden temperature drop. As a consequence of cooling, the paraffin molecules precipitate forming a gel-like structure that can obstruct the tube completely. Under these conditions, the pressure required to startup the flow can be higher than usual operating pressure. Hence, the correct prediction of the minimum pressure required is of great importance. The aim of this work is to investigate the startup flow of yield stress materials being displaced by a Newtonian fluid in pipes. To this end, a flow-loop was developed and rheological tests were performed. Aqueous solutions of Carbopol® were used as a model fluid to mimic the waxy crude oil viscoplastic behavior. A polyester resin pipe with a threaded inner wall was used in order to avoid wall slip. The test section was validated with Hagen-Poiseuille law and with a Herschel-Bulkley analytical result. Applying different entrance pressures rates, the minimum pressure required to restart the flow was identified. An elasto-viscoplastic thixotropic behavior was observed through a delayed start up and the avalanche effect. Moreover, an increase in dimensionless entrance pressure rate results in a decrease in delay and clearance times. It is seen that the higher the viscosity of incoming fluid the lower the displacement velocity. A Herschel-Bulkley relationship between the dimensionless entrance pressure rate and the dimensionless shear stress required to start the flow is observed.