A non-isothermal approach to evaluate the impact of the cooling stage on the startup flow of waxy crude oils

Waxy crude oils submitted to low temperatures at quiescent conditions turn into a gel with a corresponding yield stress. Thus, a higher pressure drop is needed to restart, leading to a flow assurance issue. Generally, this problem is tackled by considering the material at rest, with a given state and an imposed pressure drop. In the present work, we investigate how the cooling process after a shutdown impacts the problem. We emulate a stoppage of short duration so that the cooling after the shutdown stage does not lead to a condition of uniform temperature. The temperature-dependent rheology of the material is such that the crude oil shows viscoplastic behavior only below the gelation temperature. The early stages of the process, before and during the shutdown, resulting in a heterogeneous gel, which serves as an initial condition for the restart stage. We perform 2D non isothermal simulations in a pipe considering three stages, namely (i) normal operation conditions; (ii) cooling stage at rest after stoppage; (iii) restart by an imposed pressure drop. Particular emphasis is given to the role of the plastic number (Pl) and the duration of the cooling after the shutdown stage. A comparison with the corresponding homogeneous condition is also made. The results show that higher values of Pl and higher waiting times in the cooling after the shutdown stage lead to conditions less prone to restart. Only cooling times of short duration lead to restart in the same conditions as the normal operation ones. The pressure drop necessary for a restart is an increasing function of the cooling time.

Automated summary

Waxy crude oils can turn into a gel at low temperatures, causing flow assurance issues during restart due to the higher pressure drop required. This study investigates the impact of cooling process after shutdown on the problem. The results show that the plastic number and the duration of cooling after shutdown play a role in the restart conditions, with higher plastic numbers and longer cooling times leading to more difficult restarts.