Pressure propagation and flow restart in the multi-plug gelled pipeline

An understanding of pressure propagation mechanisms is crucial in finding suitable conditions for the temporary storage, transmission and attenuation of a large pressure signal in a multi-plug gelled pipeline. The current work investigates the transient flow start in a multi-plug gelled pipeline, filled with a weakly compressible, thixotropic waxy gelled crude oil. A multi-plug gel can form either naturally or artificially. We solve time-dependent mass and momentum conservation equations for the gas phase and conservation equations together with a shear-history-dependent constitutive model for the gel phase to capture the transient phenomena. An advection equation traces the motion of the gel-gas interface using the volume of fluid method. The results of this work show that pressure propagation in the first gel plug compresses it. The compressed first gel plug further compresses the gas pocket, which delays pressure propagation in the downstream gel. The delay in the pressure propagation through the second gel plug creates a high-pressure gradient in the first gel plug. A high-pressure gradient in the first gel results in faster degradation of the first gel. Degraded gel offers a substantially lower resistance to flow, allowing a high-pressure gradient in the second gel. A steeper pressure gradient assists the breakage of the next gel plug and results in a sequential gel breakage. This sequential gel breakage allows flow restart in a long pipeline. The results also indicate that the compressibility of the gas pocket dominates the overall compressibility of the system, controlling both pressure propagation and flow-restart time.

Automated summary

Understanding pressure propagation mechanisms in multi-plug gelled pipelines is crucial for temporary storage, transmission, and attenuation of pressure signals. The study investigates transient flow start in such pipelines, revealing the significance of pressure gradients between different plugs. The compressibility of the gas pocket plays a key role in controlling overall compressibility and flow-restart time.