Simulation of expansion driven flow instabilities in long risers

Abstract

A series of former small scale experiments on unstable gas lift conducted at the NTNU multiphase laboratory have become subject to numerical simulations with two different multiphase one-dimensional flow simulators. In the experimental setup the injected gas accumulates in a riser inlet bend configuration and then discharges into the riser due to expansion driven flow acceleration in the riser. This gives a cyclic flow, resulting in unstable production with characteristics similar to those associated with density wave instability. No similar experimental contributions have been found in the literature. Simulations with and without slug tracking have been performed with the commercially available simulator OLGA. The simulations without slug tracking have been conducted on a fine grid with a cell length in the order of one pipe diameter, thus representing a slug capturing approach. The OLGA simulations with slug tracking have been conducted on a coarser grid. The SLUGGIT simulator is a non-commercial in-house NTNU slug tracking simulator based on a moving grid formulation. This simulator exists in several versions which are all implemented in C++ by use of object oriented techniques. One of these versions has been modified to handle a gas source at any location along the pipeline. Both OLGA without slug tracking as well as the SLUGGIT have proven capable of reproducing the unstable expansion driven flows shown inthe experiments.