A New Mechanism for Flow Instabilities in Gas-Condensate Fields at Low Production Rates

The number of gas-condensate fields nearing the end of their operational life and producing less is growing, making the prediction and management of instabilities associated with low production rates increasingly critical. Instabilities are commonly assumed to originate in the riser or riser base. However, recent simulations of a 45-km gas-condensate pipeline in the Åsgard Field indicate a different source. The flow model used is based on data gathered in the Equinor EX-certified test laboratory in Porsgrunn (PLAB), using "real fluids" designed to imitate the properties of the Åsgard fluids. The simulations reveal massive water/monoethylene glycol (MEG) surge waves traveling through the pipeline. The surge waves start out relatively small near the pipeline inlet and grow substantially in length, reaching around 4 km near the riser base. The surge waves persisted in the simulation after replacing the riser with a pressure and temperature node, eliminating riser feedback as an indirect cause of the instabilities. The surge waves are primarily composed of water/MEG, which they transport through the pipeline, pushing a large volume of condensate ahead. New surge waves appear every 2 to 6 hours. There is no net water/MEG transport between the waves, resulting in intermittent water/MEG flow at the outlet. The surge waves emerge from the liquid accumulation in the pipeline's low points and the gravity-dominated water/MEG roll wave flow in uphill sections. The Kelvin-Helmholtz instability criterion predicts the roll waves. In this study we examine whether Kelvin-Helmholtz instabilities and roll waves are prerequisites for the onset of surge waves in gas-condensate pipelines.